lab/lab2/

mod.rs

//! Welcome to Lab 2! The goal of this lab is to use the RPC service we built in
//! Lab 1 as the basis to impelement a scalable Tribbler infrastructure
//!
//! ## Lab 2
//!
//! Welcome to Lab 2. The goal of this lab is to use the RPC service we built in
//! Lab 1 as the basis to impelement a scalable Tribbler infrastructure, split
//! into separate front-ends and back-ends by implementing the [new_bin_client],
//! [serve_keeper], and [new_front] functions
//!
//! In particular, you will do this by first wrapping the key-value server into
//! a distributed key-value service called *bin storage*. Your implementation
//! should fit the [tribbler::storage::BinStorage] trait. Then, on top of this
//! storage layer, you will implement a stateless Tribbler front-end that fits
//! the [tribbler::trib::Server] trait.
//!
//! ## Get Your Repo
//!
//! As mentioned earlier, we will send out an invitation link on Piazza/Canvas.
//! Once available, access it and follow the instructions provided in the
//! repository to get started. Each lab comes with its own starter code, so you
//! will need to copy your solution for Lab 1 into Lab 2 repository, and start
//! working from there.
//!
//! ## System Architecture
//!
//! The overall system architecture looks like this:
//!
//! ![System Arch](https://cseweb.ucsd.edu/classes/sp21/cse223B-a/arch.png)
//!
//! Our system is split into two parts. The upper part is the Tribbler service.
//! The lower part is a general purpose distributed key-value store called "Bin
//! Storage". Users' web browsers make AJAX calls to the Tribbler front-ends.
//! These front-end servers then convert those requests into one or more calls
//! to the bin storage client interface. This interface then uses RPC to talk to
//! one or more of the simple key-value stores that we built in Lab 1.
//!
//! Separately, there is also a `keeper` running in the background that
//! maintains the bin storage in a coherent state. Both the bin storage clients
//! and the keeper use the same RPC interface (the one developed in Lab 1) to
//! communicate with the back-ends. The interface semantics defined in
//! [tribbler::storage::Storage] should be sufficient to perform all the bin
//! storage tasks without any changes.
//!
//! ## Bin Storage
//!
//! **Bin Storage** is a distributed key-value system that combines a bunch of
//! [tribbler::storage::Storage] back-ends and presents itself as a unified and
//! scalable storage service over the network. Bin storage service is provided
//! via the [tribbler::storage::BinStorage] trait.
//!
//! Conceptually, bin storage provides an infinite set of separate
//! [Storage](tribbler::storage::Storage) instances called *bins*. Each bin has
//! a different name, and a caller can fetch a bin via the
//! [BinStorage::bin()](tribbler::storage::BinStorage) function. This is the
//! only function in the [BinStorage](tribbler::storage::BinStorage) trait.
//! Initially, all bins are empty.
//!
//! The bin storage service provides this service on top of a finite set of
//! back-end key-value stores (Lab 1). Each bin (a virtual KV store) will be
//! mapped to one of the key-value back-ends. Thus, this is a many-to-one
//! mapping, with multiple bins potentially sharing a single back-end. You will
//! do this by splitting the key space with prefixes (or suffixes) that encode
//! the bin names. As an example, the figure below shows a bin storage hashing 6
//! bins into 3 back-ends, where each bin is indicated with a different prefix.
//! When a caller calls `get("some-key")` on the bin `"alice"`, for example, the
//! bin storage client translates this into a `get("alice::some-key")` RPC call
//! on back-end 0.
//!
//! ![Bin Storage](https://cseweb.ucsd.edu/classes/sp21/cse223B-a/bins.png)
//!
//! In addition to key-value stores in bins, the bin storage also maintains a
//! coarsely synchronized logical clock across *all* the bins, so items in
//! different bins can be reasonably ordered relative to each other. Since the
//! back-ends are "dumb", and do not talk to each other, this clock
//! synchronization is a job for the background keeper process.
//!
//! Note that the clocks are NOT real-time synchronized (which would be too
//! expensive for a distributed system), but only coarsely synchronized. In
//! particular, if a `clock()` call is issued at least 3 seconds after another
//! `clock()` call, no matter which bins they are issued in, bin storage always
//! guarantees that the later call always returns a clock value no smaller than
//! the earlier one's. Note that a bin storage with one single back-end will
//! trivially satisfy this requirement. The keeper's job comes in when there are
//! multiple back-ends to keep in sync.
//!
//! As mentioned, we already implemented the back-end for Lab 1, and the
//! key-value store API will not change. Both the bin storage client and the
//! keeper will communicate with the "dumb" back-ends via the RPC calls we
//! implemented, and cooperatively they will present a coherent bin storage
//! interface to upper layer applications. Since we'll be reusing the work from
//! Lab 1, make sure you've implemented the back-end correctly!
//!
//! ## Tribbler
//!
//! Before we look into how the Tribbler service works, let's first define what
//! a tribble is.
//!
//! A [Tribble](tribbler::trib::Trib) is a struct that has 4 fields:
//!
//! ```rust
//! pub struct Trib {
//!     /// who posted this trib
//!     pub user: String,
//!     /// the content of the trib
//!     pub message: String,
//!     /// the physical timestamp when posted
//!     pub time: u64,
//!     /// the logical timestamp when posted
//!     pub clock: u64,
//! }
//! ```
//!
//! `time` is a real-world timestamp, read from the machine's time on the
//! front-end immediately after `post()` is called on a
//! [Server](tribbler::trib::Server). In order to sort tribbles in a globally
//! consistent and *reasonable* order, however, we cannot sort the tribbles only
//! by this timestamp, as different front-ends may have different clocks. For
//! sorting, the Tribbler service uses a distributed logical `clock` in `u64`.
//!
//! When sorting many tribbles into a single timeline, you should sort by the
//! fields following this priority:
//!
//!  1. `clock` The logical timestamp.
//!  2. `time` The physical timestamp.
//!  3. `user` The user id.
//!  4. `message` The message content.
//!
//! We call this the *Tribble Order*.
//!
//! ## Tribbler Service Interface
//!
//! The Tribbler service logic is defined by the
//! [Server](tribbler::trib::Server) interface (in `tribbler/src/trib.rs`). This
//! is how the web user interface interacts with a Tribbler server.
//!
//! --------------
//!
//! ```rust
//! async fn sign_up(&self, user: &str) -> TribResult<()>;
//! ```
//!
//! Creates a new user. After a user is created, it will never disappear in the
//! system.
//!
//! Returns error when the username is invalid or the user already exists. If
//! there are concurrent sign-ups, both sign-ups may succeed.
//!
//! Valid usernames must be no longer than
//! [MAX_USERNAME_LEN](tribbler::trib::MAX_USERNAME_LEN) characters but not
//! empty, must start with a lower-case letter, and can only contain lower-case
//! letters or numbers. There is a helper function called
//! [is_valid_username](tribbler::trib::is_valid_username) which you can use to
//! check if a username is valid.
//!
//! ---
//!
//! ```rust
//! async fn list_users(&self) -> TribResult<Vec<String>>;
//! ```
//!
//! Lists at least [MIN_LIST_USER](tribbler::trib::MIN_LIST_USER) different
//! registered users. When there are less than [MIN_LIST_USER](tribbler::trib::MIN_LIST_USER) users that have ever signed
//! up, list all of them. The returned usernames should be sorted in
//! alphabetical order.
//!
//! This is just for showing some users on the front page; this is not for
//! listing all the users that have ever signed up, as that would be too
//! expensive in a large system.
//!
//! ---
//!
//! ```rust
//! async fn post(&self, who: &str, post: &str, clock: u64) -> TribResult<()>;
//! ```
//!
//! Posts a tribble. `clock` is the maximum clock value this user client has
//! ever seen (from reading tribbles via `home()` or `tribs()`). It returns
//! error when the user does not exist or the post is too long (longer than
//! [MAX_TRIB_LEN](tribbler::trib::MAX_TRIB_LEN)`).
//!
//! ---
//!
//! ```rust
//! async fn tribs(&self, user: &str) -> TribResult<Vec<Arc<Trib>>>;
//! ```
//!
//! Lists the recent [MAX_TRIB_FETCH](tribbler::trib::MAX_TRIB_FETCH) tribbles
//! that a user posted. Tribbles needs to be sorted in the Tribble Order, and
//! also in the same order that the user posted the tribbles.
//!
//! ---
//!
//! ```rust
//! async fn follow(&self, who: &str, whom: &str) -> TribResult<()>;
//! async fn unfollow(&self, who: &str, whom: &str) -> TribResult<()>;
//! async fn is_following(&self, who: &str, whom: &str) -> TribResult<bool>;
//! async fn following(&self, who: &str) -> TribResult<Vec<String>>;
//! ```
//!
//! These are functions to follow/unfollow, check following and listing all
//! following users of a user. Users can never follow/unfollow themselves. When
//! calling with `who` equal to `whom`, the functions must return an error. When
//! the user does not exist, the functions return an error. A user's attempt to unfollow a user that they are not currently following should return an error.  If a user performs a
//! `follow()` on another user over multiple clients concurrently, only one
//! `follow()` should succeed with no errors.
//!
//! A user can follow at most [MAX_FOLLOWING](tribbler::trib::MAX_FOLLOWING)
//! users. When a user tries to follow more than
//! [MAX_FOLLOWING](tribbler::trib::MAX_FOLLOWING) users, `follow()` should
//! return an error.
//!
//! ---
//!
//! ```rust
//! async fn home(&self, user: &str) -> TribResult<Vec<Arc<Trib>>>;
//! ```
//!
//! List the recent [MAX_TRIB_FETCH](tribbler::trib::MAX_TRIB_FETCH) tribbles
//! that are posted on the user's timeline in Tribble Order. A user's timeline is the set of tribbles that are posted by that user or any user they are following.  In addition, the
//! ordering should always satisfy the following conditions:
//!
//!  1. If tribble A is posted after tribble B, and they are both posted by the
//!     same user, A always shows after B.
//!  2. If tribble A is posted at least 10 seconds after tribble B, even if they
//!     are posted by different users, A always shows after B.
//!  3. If tribble A is posted after A's poster sees tribble B, A always
//!     shows after B.
//!
//! A is *posted after* B means A calls `post()` after B's `post()` has
//! returned.
//!
//! The function returns error when the user does not exist.
//!
//! ---
//!
//! In addition to normal errors, these calls might also return IO errors if the
//! implementation is unable to communicate with a remote service. Returning a
//! `Ok()` type means that the call is successfully executed; returning an
//! `Err()` error means that the call may or may not have successfully executed
//! (depending on when/where the error occurred).
//!
//! Note that the [tribbler::ref_impl] module contains a reference
//! implementation for [Server](tribbler::trib::Server) (which you played with
//! briefly in the lab setup).
//!
//! ## Entry Functions
//!
//! Your job for Lab 2 is two-fold.
//!
//!   1. Implement the bin storage service by implementing the bin storage
//!      client logic (the [new_bin_client] function) and the keeper (the
//!      [serve_keeper] function).
//!   2. Reimplement the Tribbler service by translating all the Tribbler
//!      service calls into bin storage service calls (the [new_front]
//!      function).
//!
//! You can find these entry functions in `lab/src/lab2/lab.rs`:
//!
//! ```rust
//! pub async fn new_bin_client(backs: Vec<String>) -> TribResult<Box<dyn BinStorage>>
//! ```
//!
//! This function is similar to [crate::lab1::new_client] but instead returns a
//! type implementing the [tribbler::storage::BinStorage] trait.
//! [BinStorage](tribbler::storage::BinStorage) has only one function called
//! `bin()`, which takes a bin name and returns a
//! [Storage](tribbler::storage::Storage). A bin storage provides another layer
//! of mapping, where the caller will first gets a key-value store (the bin)
//! with a name and then performs calls on the returned store. Different bin
//! names should logically return completely separate key-value storage spaces,
//! even though multiple bins might share a single physical back-end store.
//!
//! For your convenience, we have provided a small package called
//! [tribbler::colon] which you can use for escaping all the colons in a string.
//! The escaping rule is simple, all `'|'` characters are escaped into `"||"`,
//! and all `':'` characters are escaped into `"|;"`. Please use this package
//! wisely.
//!
//! ---
//!
//! ```rust
//! pub async fn serve_keeper(kc: KeeperConfig) -> TribResult<()>
//! ```
//!
//! This function is a blocking function, similar to [crate::lab1::serve_back].
//! It will spawn a keeper instance that maintains the back-ends in a bin store
//! in consistent state. For Lab 2, there will be only one keeper and the keeper
//! won't have a lot to do. In Lab 3 we'll be adding fault tolerance. Then,
//! there will be multiple keepers, and they will handle maintaining consistency
//! as back-ends die and rejoin.
//!
//! (Note: A blocking function in this context means that it will never return
//! under normal operation. If it does return, it will be because it experienced
//! an error).
//!
//! The [tribbler::config::KeeperConfig] struct contains all the information
//! needed on back-ends, and also some additional information about other
//! keepers which you can ignore until Lab 3:
//!
//! - `backs: Vec<String>`: This is the complete list of addresses to back-ends
//!   that the keeper needs to maintain.
//! - `addrs: Vec<String>`: This is the list of addresses for keepers. Lab 2
//!   will have only one keeper; Lab 3 will always have least three keepers, but
//!   for now you can ignore this field.
//! - `this: usize`: The index of this keeper (in the `keepers` list). For Lab
//!   2, this will always be zero.
//! - `id: u128`: A non-zero incarnation identifier for this keeper, indicating
//!   when this keeper was created relative to other keepers. For Lab 2, you may
//!   ignore this field.
//! - `ready: Option<Sender<bool>>`: A ready signal channel. It works similarly
//!   to the `ready` channel in [tribbler::config::BackConfig] from Lab
//!   1. When a keeper sends `true` on this channel, the distributed bin storage
//!   should be ready to serve. Therefore, if you need to initialize the
//!   physical back-ends in some way, make sure you do it before you send a
//!   signal over `ready`. Don't forget to send `false` to `ready` if the
//!   initialization fails.
//! - `shutdown: Option<Receiver<bool>>`: A shutdown signal channel. If a
//!   shutdown channel is provided, When any message is received on this
//!   channel, shut down the keeper. If no channel is given, then the keeper
//!   should serve indefinitely.
//!
//! Although an address for each keeper is given in the `addrs` field, your
//! keeper does not necessarily need to listen on the address given. The keeper
//! address serves more like a unique keeper identifier, and identifies that a
//! keeper should be running on a given machine.
//!
//! You can add whatever functionality you want to the keeper, so long as it is
//! limited to maintaining the integrity and consistency of the bin storage, and
//! the setup is limited to the three [new_bin_client], [new_front] and
//! [serve_keeper] calls. In particular, since the keeper is logically part of
//! the bin storage service, it may understand how a bin storage client
//! translates keys. It should NOT rely on anything specific to Tribbler.
//!
//! ---
//!
//! ```rust
//! pub async fn new_front(bin_storage: Box<dyn BinStorage>) -> TribResult<Box<dyn Server>>
//! ```
//!
//! This function takes a bin storage structure, and returns an implementation
//! of [Server](tribbler::trib::Server). The returned instance then will serve
//! as a service front-end that takes Tribbler service requests, and translates
//! them into one or more calls to the back-end key-value bin storage. This
//! front-end should be stateless, concurrency safe, and ready to be killed at
//! any time. This means that at any time during its execution on any call, the
//! back-end key-value store always needs to stay in a consistent state. Also,
//! note that one front-end might be taking multiple concurrent requests from
//! the Web, and there might be multiple front-ends talking to the same
//! back-end, so make sure your system handles all the concurrency issues
//! correctly.
//!
//! Note that your tribbler service should not rely on any special features that
//! you added to your bin storage client. In particular, we should be able to
//! swap out your [BinStorage](tribbler::storage::BinStorage) for our own, and
//! your Tribbler front-end should be perfectly happy (and none the wiser). In
//! particular, this means that you cannot rely on the bin storage keeper to
//! perform Tribbler related tasks. A front-ends may spawn additional background
//! tasks if additional work is required by your implementation.
//!
//! ## Putting the pieces together
//!
//! Your first step should be to write the bin storage service. Since we might
//! have multiple parts running at the same time, potentially on different
//! machines, we need a configuration file that specifies the serving addresses
//! of the back-ends and the keeper for the distributed bin storage. By default,
//! the system expects a file named `bins.config`.
//!
//! `bins.json` is saved in JSON format, marshalling a
//! [Config](tribbler::config::Config). We have a utility program called
//! `bins-mkcfg` that can generate a `bins.json` file automatically.
//!
//! Find a directory to use as your working directory, then run:
//!
//! ```
//! $ cargo run --bin bins-mkcfg -- --backs 3
//! ```
//!
//! This will generate a file called `bins.json` under the current directory.
//! `--backs 3` means there will be in total three back-end servers. For
//! `bins-mkcfg`, there can be at most 300 back-ends and 10 keepers. However,
//! you are free to create your own `bins.json` file that has more back-ends and
//! keepers. For now, we'll use 3 back-ends and 1 keeper (the default value for
//! `--keeps`).
//!
//! With this configuration file generated, we can now launch the back-ends:
//!
//! ```
//! $ cargo run --bin bins-back
//! ```
//!
//! This will read and parse the `bins.json` file, and spawn all the back-ends
//! that have serving address on this host. Since all the back-ends we generate
//! here are on `localhost`, all three will be spawned with this command, each
//! in its own threaded runtime. You should see three log lines showing that
//! three back-ends started, but listening on different ports. Aside from
//! reading its task from the configuration file, `bins-back` isn't much
//! different from the `kv-server` program.
//!
//! After the back-ends are ready, we can now start the keeper.
//!
//! ```
//! $ cargo run --bin bins-keep
//! ```
//!
//! This process should print a message that shows that the bin storage is ready
//! to serve.
//!
//! To play with this distributed bin storage, we can use `bins-client`:
//!
//! ```
//! $ bins-client
//! (now working on bin "")
//! > bin a
//! (now working on bin "a")
//! > get a
//! Ok(None)
//! > set a b
//! Ok(true)
//! > get a
//! Ok(Some("b"))
//! > bin t
//! (now working on bin "t")
//! > get a
//! Ok(None)
//! > bin a
//! (now working on bin "a")
//! > get a
//! Ok(Some("b"))
//! ...
//! ```
//!
//! This program reads the back-end addresses from `bins.json` and can switch
//! between different bins with the `bin` command. Otherwise, it operates
//! similarly to `kv-client` from the previous lab. The default bin is the bin
//! named "".
//!
//! Once our bin storage is up and running, we can launch our Tribbler
//! front-end:
//!
//! ```
//! $ cargo run --bin trib-front -- -s lab
//! ```
//!
//! You have used this utility before. The only new thing here is the `-s lab`
//! flag, which tells it to read the `bins.json` file and use our lab
//! implementation. This will start a stateless front-end (which you implemented
//! in this lab) that connects to the back-ends specified in `bins.json`.
//!
//! Now you can open your browser, connect to the front-end machine and play
//! with your own implementation.
//!
//! If you want to use some other configuration file, use the `--config` flag.
//! It is supported in all the utilities above.
//!
//! Again, once you've completed this lab, your Tribbler implementation should
//! be able to support multiple front-ends and multiple back-ends in a nice,
//! scalable (but not fault-tolerant) way.
//!
//! ## Assumptions
//!
//! For this lab, there are a number of simplifying assumptions we'll be making.
//! Some of these assumptions are reasonable. Many of the rest we'll remove in
//! Lab 3:
//!
//! - No network communication errors will occur.
//! - Once a back-end or a keeper starts, it will remain online forever.
//! - The system will always start in the following order: the back-ends, the
//!   keeper, then all the front-ends.
//! - The [Storage](tribbler::storage::Storage) used in the back-end will return
//!   every `clock()` call in less than 1 second.
//! - In the [Storage](tribbler::storage::Storage) used in the back-end, all IO
//!   on a single process are serialized (and hence the interface provides
//!   sequential consistency). Each visit to a key (e.g. checking if a key
//!   exist, locating its corresponding value, iterating over a number of keys)
//!   will take less than 1 millisecond. Reads and writes of 1MB of data or less
//!   as a value (in a list or a string) will take less than 1 millisecond. Note
//!   that `keys()` and `list_keys()` might take a longer time to complete
//!   because it needs to scan over all the keys.
//! - Although the Tribbler front-ends can be killed at any time, the killing
//!   won't happen very often (less than once per second).
//! - All addresses passed to the keeper and backends will be valid network addresses
//!
//! Again, some of these assumptions won't stay true for Lab 3, so try to avoid
//! relying on these assumptions if possible.
//!
//! ## Requirements
//!
//! In addition to the requirements specified by the interfaces, your
//! implementation should also satisfy the following requirements:
//!
//! - When the Tribbler service function call has valid arguments, the function
//!   call should not return an error.
//! - The front-end should be stateless and hence safe to be kill at anytime.
//! - The back-ends should be scalable, and the front-end should use the
//!   back-ends in a scalable way. This means that when the back-end is the
//!   throughput bottleneck, adding more back-ends should (with high
//!   probability) mitigate the bottleneck and lead to better overall system
//!   performance.
//! - When running with back-ends on multiple machines (assuming all the
//!   back-ends satisfy the performance assumptions), every Tribbler service
//!   call should return within three seconds. Ideally much less than this.
//! - Each back-end should maintain the same key-value pair semantics as in Lab
//!   1.
//!
//! As a result, all test cases that pass for Lab 1's storage implementation
//! should also pass on your bin storage implementation for Lab 2.
//!
//! ## Building Hints
//!
//! While you are free to build the system any way that meets the requirements,
//! here are some suggestions:
//!
//! - For each service call in the front-end, if it updates anything in the
//!   back-end storage, use only one write-RPC call to commit your change. This
//!   will make sure that the call will either succeed or fail, and not end up
//!   in some weird intermediate state. You might issue more write calls
//!   afterwards, but those should be only soft hints, meaning that if
//!   subsequent writes don't succeed, the storage system is not left in an
//!   inconsistent state.
//! - Hash the tribbles and other information into all the back-ends based on
//!   username.
//! - Synchronize the logical clocks among all the back-ends every second. (This
//!   will also serve as a heart-beat signal, which will be useful for
//!   implementing Lab 3.) However, you should not try to synchronize the clocks
//!   for every post, because that will be not scalable.
//! - Do some garbage collection when one user has too many tribbles saved in
//!   the storage. Recall that no one will ever see more than the most recent
//!   100 tribbles by any other user.
//! - Keep multiple caches for the `list_users()` call when the users are many.
//!   Note that when the user count is more than 20, you don't need to track new
//!   registered users anymore.
//! - Keep a log for the users that a user follows, where each log entry is an
//!   action of `follow()` or `unfollow()`.
//!
//! ## Possible Mistakes
//!
//! Here are some possible mistakes that a lazy and quick but incorrect
//! implementation might do:
//!
//! - **Read-modify-write**: As an example, a tribbler might read a counter from
//!   the key-value store, increment it by one, and then write it back (at the
//!   same key). This will introduce a race condition among the front-ends.
//! - **Not handling errors**: A tribbler call might require several RPC calls
//!   to the back-end. It is important to properly handle *any* error returned
//!   by these calls. It is okay to tell the user that an error occurred.
//!   However, it's not a good plan to leave your back-end inconsistent.
//! - **Sorting by the timestamps first**: Again, the Tribble Order dictates
//!   that the logic clock is the first field to consider on sorting.
//! - **Misusing the clock argument in Post()**: For example, you might directly
//!   use that argument as the new post's clock field. Intuitively, the clock
//!   argument tells the *latest* tribble a user has seen (which might be 0 if
//!   the user hasn't seen any tribbles yet), hence the new posted tribble
//!   better have a clock value that is at least larger than the argument.  You
//!   might need to do more than increment it by one though.
//! - **Generating the clock from the timestamp**: While 64-bit clock can cover
//!   a very wide time range even in the unit of nanoseconds, you should keep in
//!   mind that the front-ends are running on different servers with arbitrary
//!   physical time differences. It is not wise to generate the logical *clock*
//!   from the physical *time*.
//! - **Not handling old tribbles**: Note that only the most recent 100 tribbles
//!   of a user matter. Not handling old tribbles might lead to worse and worse
//!   performance over time and eventually break your performance promises.
//!
//! ## `Report.md`
//!
//! Please include a `Report.md` file. `Report.md` should be a text file
//! (ideally in [Markdown](https://daringfireball.net/projects/markdown/syntax)
//! format) that describes how your system works at a high level. You should be
//! able to describe how your Bin Storage works in a few sentences, and Tribbler
//! in a few more.
//!
//! The purpose of this document is two-fold:
//!  - If your `Report` is well written, it'll make our job of grading
//!    *considerably* easier.
//!  - An overview document forces you to think about your overall strategy, and
//!    forces you to try to find an elegant solution (that you can describe
//!    succinctly).
//!
//! This `Report` *will* count towards your grade, but *will not* be a
//! significant number of points overall. It's worth spending some time on, but
//! NOT at the expense of good, working code. You'll primarily be graded on
//! clarity and coverage of the big ideas of your solutions.
//!
//! ## Turning In Your Code
//!
//! Instructions for turning in the assignment are provided in the lab
//! repository.
//!
//! ## Happy Lab 2!
//!
mod lab;
pub use crate::lab2::lab::new_bin_client;
pub use crate::lab2::lab::new_front;
pub use crate::lab2::lab::serve_keeper;