Introduction to Recovery
When making progress -- it is always important to capture progress. This way, it it is ever interrupted, all is not lost. This can be done, for example, by periodic "checkpointing", by event "logging", or continuous replication (via, for example, 2PC messaging).
Maintaining geographically diverse replicas provides a way to secure against usual system failures, as well as physical hazards that may effect one location, such as natural phenomena (earth quakes, fires, etc), and those created by hiumans (bulldozer meets fiber lines). But, of course, doing this is often a challenge, not only because it might be resource intensive and financially costly, but becuase ti does require synchronizing the replicas to the level of consistency required by the application.
Checkpointing exists when the important state of the application is periodically stored, for example, into an online file system or offline tape system. When you or I backed up our computers to tape or DVD, we created a checkpoint. A more refined checkpoint might back up only the most critial state. Or, it might make a "full backup" only periodically, and record only changes in a "differential" backup in between full backups.
The problem with checkpointing is that they required a "freeze". Since backups take time, they becom blurry if the system is making process during the time the backup is being made. The backup can capture inconsistent state. This is a classic problem, for example, in attempts to backup running databases or file systems. This "freeze" can reduce availability and can be disruptive to users. This situation becomes even worse in distributed systems where the "freeze" must be across the entire system, rather than across any one host. Checkpointing systems with significant state can also be quite slow.
The alternative is to try to get luck and find a consistent recovery line across uncoordinated checkpoints. The best I canoffer there is there is always one consistent recovery line to be found -- "start". Gulp.
Another technique is to log events on each system. The classic technique is Write Ahead Logging (WAL). In this case, events are opened with an entry in a log, completed, and then closed with an entry in the log. This enables the state of an event to be more easily determined upon recovery (was it started? do we know that it finished?).
Making use of logs can be challenging, because they need to be played back. This can take a very long time. One approach is to use logging between checkpoints. Then, log entries before the checpoint aren't needed.
Beyond this, logs would ideally need to be written atomically to persistent storage. This is known as synchronous logging. This is very, very slow. And, it encounters the overhead of writing for each log entry. It is most ofen cost prohibitive. Instead, asynchronous logging is used. Log entries are buffered in memory until an entire block worth of buffer is filled, then the block is written out. This reduces and amortizes the overhead -- but leaves the log entries vulnerable to loss.
And, we haven't yet talked about how we get fast writes, or how we even know if a devices has "really" written something.
Our examples above assumed that messages would be logged by the receiver. Most of our discussion of logging will, in fact, be focused on receiver-based techniques. But, before we dive into these, let's take a quick look at logging messages at the sender.
Sender based logging is very important in those cases where receivers are thin or unreliable. In other words, we would want to log messages on the sender if the receiver does not have the resources to maintain the logs, or if the receiver is likely to fail. This might be the case, for example, if the sender is a reliable server and the receiver a portable mobile device.
So, let's assume that each sender logs each message it sends and that the receiver logs nothing. Recovery isn't quite so easy as having each sender play back its logs. Although each sender can play the messages back in the same order in which they were dispatched, there is no way to order the messages among the senders.
One solution to this problem is to follow the following protocol:
- The sender logs the message and dispatches it.
- The receiver receives the message and ACKs it with the current time (local to the receiver).
- The sender adds the timestamp contained in the ACK to its log entry to the message -- the message is now fully logged
If the above protocol is followed, the timestamp can be used to ensure that messages from multiple servers are processed by the receiver in the proper order. This is because all of the timestamps were assigned by the client, so clock skew is not a problem. (The timestamp can be as simple as a receive sequence number).
But there is one small problem. Consider Sender sending a Message to Receiver. Now consider the same Sender sending a message, m', to Receiver. If the Receiver fails before it ACKS m, it is unknown whether or not m was received by the Receiver before the Sender dispatched m' -- we can't establish whether or not a causal relationship exists.
One solution to this problem is to require that the sender send an ACK-ACK to the receiver. If the receiver blocks until it receives the ACK-ACK, the order of the messages will be clear.
Given this protocol, recovery is very, very straight-forward. Upon reinitialization, the failed receiver sends a message to all other hosts in the system with the number of the last message that it remembers ACKing. The senders then replay their logs, including the receiver-assigned sequence numbers. The receiver applies these messages in the order of their sequence number. The senders know to ignore the responses to these messages. This process leads to a very simple, uncomplicated recovery.
Recovery from Failure
As you know, distributed systems are very likely to suffer from failure. This is a characteristic of their scale. Sometimes we have sufficient redundancy to keep failures transparent. On other occasions, we need to repair or replace failed processors and pick up where we left off before the failure. This process is known as recovery.
Needless to say, recovery is a very important component of many real-world systems. Recovery usually involves checkpointing and/or logging. Checkpointing involves periodically saving the state of the process. Logging involves recording the operations that produced the current state, so that they can be repeated, if necessary.
Let's assume that one system fails and is restored to a previous state (this is called a rollback). From this point, it will charge forward and repeat those things that it had done between this previous state and the time of the failure. This includes messages that it may have sent to other systems. These repeated messages are known as duplicate messages. It is also the case that after a rollback other systems may have received messages that the revovering system doesn't "remember" sending. These messages are known as orphan messages.
The other systems must be able to tolerate the duplicate messages, such as might be the case for idempotent operations, or detect them and discard them. If they are unable to do this, the other systems must also rollback to a prior state. The rollback of more systems might compound the problem, since the rollback may orphan more messages and the progress might cause more duplicates. When the rollback of one system causes another system to rollback, this is known as cascading rollbacks. Eventually the systems will reach a state where they can move forward together. This state is known as a recovery line. After a failure, cooperating systems must rollback to a recovery line.
Another problem involves the interaction of the system with the real-world. After a rollback, a system may duplicate output, or request the same input again. This is called studdering.
One very, very common technique involves the use of incarnation numbers. Each contiguous period of uptime on a particular system is known as an incarnation of that system. Rebooting a system or restarting a cooperating process results in a new incarnation. These incarnations can be numbered. This number, the incarnation number, can be used to eliminate duplicate messages. It must be kept in stable storage, and incremented for each incarnation.
When a system is reincarnated, it sends a message to the cooperating systems informing them of the new incarnation number. The incarnation number is also sent out with all messages.
The receiver of a message can use the incarnation number as follows:
- If the incarnation number in the message is less than the expected incarnation number, the message is a duplicate, so it should be discarded.
- If the incarnation number in the message is greater than the expected incarnation number, the sender is recovering, so block accepting messages, until it informs us about its new incarnation number.
- If the incarnation number is the same as the expected incarnation number, accept the message.
One approach to checkpointing is to have each system periodically record its state. Even if all processors make checkpoints at the same frequency, there is no guarantee that the most recent checkpoints across all systems will be consistent. Among other things, clock drift implies that the checkpoints won't necessarily be made at exactly the same time. If checkpointing is a low-priority background task, it might also be the case that the checkpoints across the systems won't necessarily be consistent, because the systems may have cycles to burn at different times or with a completely different frequency.
In the event of a failure, recovery requires finding the recovery lines that restores the system as a whole to the most recent state. This is known as the maximum recovery line.
An interval is the period of time between checkpoints. If we number checkpoints, C1, C2, C3, C4, &c., the intervals following each of these checkpoints can be labeled I1, I2, I3, and I4, respectively. it is important to note that the intervals need not be the same length.
If we have multiple processors, we can use subscripts such as Ci,c and Ii,c, where i is the processor number and c is the checkpoint sequence number.
When a processor receives a message, that message usually causes it to take some action. This implies that the processor that receives a message is dependent on the processor that sent the message. Specifically if a processor receives a message during an interval, it is dependent on the interval on the sender's processor during which the message was sent. This type of dependency can cause cascading rollbacks.
In the example below, Ik,2 depends on Ij,1
If we consider the messages sent among systems we can construct an Interval Dependency Graph (IDG). If any intervals are removed from the graph due to rollbacks or failures, we must remove all intervals that they reference -- this is a transitive operation.
The graph is constructed by creating a node for each interval, and then connecting subsequent intervals on the same processor by constructing an edge from a predecessor to its successor. Then an edge is draw from each interval during which one or more messages were received to the interval or intervals during which the message(s) was or were sent.
The edge from one interval to its successor on the same processor exists to ensure that we can't develop "holes" in our state -- a hole would imply wasted checkpoints -- those before the hole would be useless.
The edge from a receiver to the sender shows the dependency of the sender on the receiver. Remember that the arrow goes the opposite way in the IDG than it did when we showed the message being sent earlier -- this is because the sender is dependent on the receiver, not the other way around. If other actions generate dependencies, they can be represented the same way.
Where is this graph stored? Each processor keeps the nodes and edges that are associated with it.
How Do We Find The Recovery Line?
Very simply. Upon recovery, we tell other processors what checkpoint we'll be installing. Then they rollback to an interval independent of the lost intervals and broadcast a similar message. This continues until the recovery line is established.
We can decrease the number of rollbacks necessary to find a recovery line by coordinating checkpoints. We'll discuss two methods for doing this.
Recording Message Sequence Numbers
If messages contain sequence numbers, we can use them to keep track of who has sent us messages since our last checkpoint.
Each time we make a checkpoint, we send a message to each processor that has sent us messages since the last time we checkpointed -- we depend on these processors.
When these processors receive our message, they check to see if they have checkpointed since the last time they sent us a message. If not, they create a checkpoint, to satisfy our dependency in the event of a failure.
Another method for coordinating checkpoints is to take advantage of synchronized clocks, if available. Each processor creates a checkpoint every T units of time. Since even synchronized clocks may have a skew, we assign a sequence number to each checkpoint. This sequence number is sent in all messages. If a processor discovers that the sender has checkpointed more recently, it creates a checkpoint. This checkpoint should be made before passing the message up to the application -- this ensures that we remember the message in the event of a failure, so the sender won't need to rollback and resend.
Introduction to Logging
So far our discussion of failure and recovery has centered around checkpointing the state of the system. The next technique we will discuss is logging. Instead of preserving the state of the entire system at once, we will log, or record, each change to the state of the system. These changes are easy to identify -- we just log each message. In the event of a failure, we will play back this log and incrementally restore the system to its original state.
The most intuitive technique for logging a system is synchronous logging. Synchronous logging requires that all messages are logged before they are passed to the application. In the event of a failure, recovery can be achieved by playing back all of the logs. Occasional checkpointing can allow the deletion of prior log entries.
This approach does work, but it is tremendously expensive. Messages cannot be passed to the application, until the messages are committed to a stable store. Stable storage is usually, much slower than RAM, &c. Sometimes performance can be improved by carefully managing the I/O device, e.x. using one disk for the log file to avoid seek delays, but it is still much slower than astable storage.
In order to avoid the performance penalty associated with synchronous logging, we may choose to collect messages in RAM and to write them out occasionally as a batch, perhaps during idle times. If we take this approach, we have a similar management problem to that of uncoordinated checkpoints. Some messages may be logged, while others may not be logged.
One Approach to Recovery Using Asynchronous Logging
In order to recover from failure given asynchronous logging, we need some way for the processors to keep track of their current state, some way for them to communicate it to each other, and some way of evaluating the state of the system.
The approach that we'll discuss makes use of direct dependency vectors. Before defining a DDV, we need to define an interval. An interval on a processor is the period of time between receiving two messages. The intervals on a particular processor are sequentially numbered and given an interval index. Each time a processor sends a message, it also sends its current interval.
On the receiving process, a direct dependency vector is associated with the event. This vector contains the processor's understanding of the interval on all processors. This information may be out of date, and does not take gossip into account. One processor is only informed about another processor's interval, if it directly receives a message from that processor. It is important to pay attention to the fact that the interval must be directly communicated from one process to another -- it is not transitive and cannot be communicated indirectly. A message from processor X to processor Y only contains X's interval, not the DDV present on X.
The diagram below illustrates the intervals and DDVs associated with each processor.
We can define the global state of the system at any point in time to be the collection of the DDVs at each processor. We can organize this state into a Global Dependency Matrix (GDM). We'll follow your textbook's nomenclature and give this the somewhat misleading label GDV.
The diagram below shows the GDV at two different points in time:
Notice that the first GDV is inconsistent. The DDV component of the GDV from Processor 1 indicates that Processor 1 has received a message from Processor 2 indicating that Processor 2 is in Interval 2. But the DDV vector within the GDV from Processor 2 indicates that Processor 2 is in Interval 1. This indicates an inconsistent recovery line. The second GDV does not have any inconsistencies -- it is consistent recovery line.
We can check to see if a GDV is valid, by looking at each DDV one at a time. For each entry in a consistent DDV, the interval shown for another procesor must be less than or equal to the interval that that processor's DDV within the GDV contains for itself.GDV(p)[q] <= GDV(q)[q], 1<=p, q<=M
In other words, no processor can have received a message from another processor originating in an interval in advance of that processor's current interval.
So, we know what an inconsistent state looks like, but how do we find a recovery line? We start out with the initial state of all processors composing a valid recovery line. We store this in a vector, RV, that contains the interval index for each processor -- these will initially be 0.
Then each time a processor commits enough log entries or checkpoints to make a new interval stable, it sends this information to a central server that tries to advance the recovery line.
To find the recovery line, the coordinator keeps a matrix with the current recovery line, and a bin of recent interval updates that are are not a part of the current recovery line. The coordinator then applies the recent update to the matrix and checks for consistency. It also attempts to include as many of the other recent updates from the bin as possible. If it is unable to incorporate the most recent update into a consistent recovery lines, it adds it to the bin -- a future update may make it usable.
For those who would like a more formal look at one possible implementation of this algorithm, the following pseudo-code might be helpful:
find_recoverable (RV, p, k) TRV = RV // Make a copy of the current recover line (RV) TRV[p] = k // advance the interval for processor p to k // in this temporary copy -- a candidate // that needs to be check for inconsistency // Compare the DDV for each processor to the entries in TRV, // a temporary and optimistic GDV. Record inconsistencies // in Max for q = 1 to M // M is the number of processors Max[q] = max(TRV[q], DDV(p,q)[q]) // Try to resolve inconsistencies by looking at stable intervals // that we haven't been able to use, yet (previously other // processors weren't stable in sufficiently advanced intervals) While there is a q such that Max[q] > TRV[q] Let l be the minimum index such that l >= Max[q] and stable(l) If no such l exists // if we can't fix the problem with these states return (fail) // we're going to have to try again later to advance TRV[q] = l // If we can fix this problem, optimistically // add this interval to the TRV for r = 1 to M // And recompute the Max matrix so that we // can check for new incocnsistencies Max[r] = max(Max[r], DDV(q,l)[r]) // There are no inconsistencies, sThere are no inconsistencies, // so make the candidate recovery line, the real thing. RV = TRV return (success)
One might observe that it isn't necessary for a processor to log every message. It only needs to log those messages that have originated from processors that have taken checkpoints more recently than it has. If processors with less recent checkpoints were to fail, they would be forced to roll back prior to the message's sending, anyway.
One approach for this problem is for each processor to give a sequence number to its checkpoints (as we have done before), and to keep a vector containing its best understanding of the most recent checkpoints on all other processors.
If each time a message is sent, the current recovery line (CRL) is sent with it, a processor can determine if it is ahead of, or behind, the sender with respect to making checkpoints by comparing its checkpoint sequence number, to the sequence number of the sender in CRL received with the message. If the sender is ahead, the receiver will log the message. If not, it won't worry about it.
Much of today's discussion follows chapter 13 of the Chow and Johnson textbook. Additionally, the figures used in class today were based on those in this book. The bibliographic information is as follows:Chow, R. and Johnson, T., Distributed Operating Systems & Algorithms, corrected 1st edition, AWL, 1998, ISBN: 0-201-49838-3.