On Wednesday we kicked off a new set of papers in the reading group. We have started with one of the classical foundational papers in distributed systems and looked at the Chandy-Lamport token-based distributed snapshot algorithm. The basic idea here is to capture the state of distributed processes and channels by “flushing” the messages out of the channels with markers. The markers ensure the causality if not broken, despite the processes taking their local snapshots at different times (and with no affinity to the physical time). I am not going to summarize the paper, as there is plenty of material on the internet on the subject, however, here is our group’s short presentation by Maher Gamal:
1) Use of snapshots. Much of our discussion focused on the use of snapshots. Aside from the trivial use for disaster recovery, snapshots are useful for debugging and runtime verification. The paper suggests some debugging/monitoring usage, like detecting stable properties of the algorithms. However, we also think that detecting violations of certain properties may be more useful in the real world. For instance, detecting the violations of invariant properties at runtime.
Just last week we talked about Aragog, a system for runtime verification of network functions. And while it does not directly use snapshots, it relies on time synchronization to make sure that the messages come from different consistent cuts of the state, and the cause and effect relationship play out correctly in the constructed state machine.
2) Snapshots of states that did not happen. Interesting things about the Chandy-Lamport snapshots are that they may capture a system state that did not happen in the execution. This is because the snapshots are taken progressively as the markers propagate through the channels, and essentially the snapshot gets rolled out at the communication speed.
3) Timely snapshots. We also brought up snapshots that are close to the wall clock. These may be more useful for debugging purposes, than Chandy-Lamport, as they provide some notion of when things happened. Additionally, more tight snapshots that are taken at about the same time globally are better at recording true state (or should we say has fewer timing artifacts?)
Our reading group takes place over Zoom every Wednesday at 2:00 pm EST. We have a slack group where we post papers, hold discussions, and most importantly manage Zoom invites to paper discussions. Please join the slack group to get involved!
Earlier I briefly mentioned Retroscope, our distributed snapshot library that makes taking non-blocking, unplanned consistent global distributed snapshots possible. However, these snapshots are only good if we know how to use them well. Of course the most obvious use case is just a data backup, and despite it being an important application for snapshots, I feel it being a bit boring to my taste. What I am thinking right now is using snapshots for distributed monitoring and debugging.
Let’s consider an application that has a global invariant predicate P, and we want to check if a distributed system holds the invariant P at all times. This means that we should never see a consistent cut in which predicate P = false. So our problem is boiling down to looking for consistent cuts that violate P. Luckily, Retroscope can do exactly this, since we can take one snapshots and incrementally move forward in time as the application execution progresses, checking the predicates by looking at consistent cuts as the state advances.
With the basic Retroscope described in the earlier post, finding predicate violations is a rather cumbersome effort that requires writing new code for every invariant a user wishes to check. So in the past few months I have been working on Retroscope extension tailored specifically for debugging and monitoring use cases. Improved Retroscope exposes the Retroscope Query Language (RQL), a SQL-like interface to allow users write queries to search for conditions happening in the consistent cuts.
Now let’s go back to our hypothetical system with global invariant P and for now assume P holds when all local predicates p0, p1, p2, …, pn hold on the nodes [0 … n]. As such, P = p0∧ p1∧ p2∧ … ∧ pn, and if any of the local predicates fail, the global predicate fails as well. For the simplicity of the example, we can say that local predicate pi is following: pi = ai + bi > ci. This makes each node maintain all three variables, although the nodes may have different values. With Retroscope, we can expose these local variables to be stored in the local log named inv. The log will maintain both the current version of the variables and the history of variable changes.
How do we look for the violation of such invariant with RQL? Just a single query would suffice for us:
SELECT inv.a, inv.b, inv.c FROM inv WHEN (inv.a + inv.b <= inv.c) LINK SAME_NODE
Now we can dissect this query into bits and see what happens there. RQL queries are meant to retrieve consistent cuts that satisfy certain criteria. The list of parameters following the SELECT statement specifies what variables we want to see in the resultant consistent cuts. FROM keyword enumerate the logs we use in this particular query. The actual consistent cut criteria are specified after the WHEN keyword. In the particular case the condition for emitting cuts is (inv.a + inv.b <= inv.c) LINK SAME_NODE, which is equivalent to emitting cuts when the following holds:
By now a curios reader would have probably asked a question of why we even bother with consistent cut in this particular example. All predicates can be checked locally and their evaluation does not depend on other remote servers, so we can simply run local monitors and do not worry about consistent cuts and time synchronization at all: failure on one node designate the failure of the system globally no matter the time. Retroscope and RQL shines when we break away from this locality. What if our invariant involves messages being sent and received? Or what if in involves different parameters that exists on different machines at the same time? With the ability of looking at consistent cuts, RQL breaches the boundary of a single node. Below I list just a few variations of the original query that no longer deal with conjunction of local predicates and look at global state as a whole:
SELECT inv.a, inv.b, anv.c FROM inv WHEN inv.a + inv.b <= inv.c
Omitting LINK SAME_NODE part changes the operation of the query drastically, as all three variables are no longer bound to co-exist on the same node:
SELECT inv.a, inv.b, anv.c FROM inv WHEN (inv.a + inv.b <= inv.c)LINK EACH_NODE
Replacing LINK SAME_NODE with LINK EACH_NODE, changes the search condition to require every node satisfying it in the consistent cut:
SELECT inv.a, inv.b, anv.c FROM inv WHEN (inv.a + inv.b) LINK SAME_NODE <= inv.c
Rewriting the condition to WHEN (inv.a + inv.b) LINK SAME_NODE <= inv.c will cause the inv.a and inv.b to be summed on the same nodes, and compared to inv.c values from other nodes as well, so the consistent cut is emitted when
SELECT inv.a, inv.b, anv.c FROM inv WHEN (inv.a + inv.b <= inv.c) AND NODE($1) = NODE($3)
This query restricts inv.a and inv.c to be on the same node. $1 is the placeholder for the first variable encountered while parsing left to right, and $3 is the third variable. This emits the consistent cuts when
Above are just a few simple examples of what is possible with RQL, however there are limitations. The biggest limitation is the complexity of the conditions. Even though RQL does not limit how many operations are possible in the condition of the query, having large expressions can slow the system down drastically. For example, a simple WHEN inv.a > inv.b will examine all a’s that exist on the nodes of the system at the consistent cut and all b’s in every possible combination. For . Comparison is then carried out on every element of product set E.
P.S. I illustrated some of the syntax as it operates at the time of this writing, however RQL is developing, and I am not sure I like syntax of conditions too much, so it is a subject to change.
Taking a consistent snapshot of a distributed system is no trivial task for the reasons of asynchrony between the nodes in the system. As the state of each machine changes in response to incoming external messages or internal events, each node may produce a log of such state changes. With the log abstraction, the problem of taking a snapshot transforms into the issue of aligning the logs together and finding a consistent cut among all these logs. However, time asynchrony between the servers makes collating all the system logs difficult using just physical clocks available at each machine, because clocks tend to drift, producing some time asynchrony or time uncertainty. Various time synchronization protocols, such as NTP and PTP, exist, but perfect synchronization is still unattainable.
Retroscope is the system we designed to take unplanned, non-blocking, consistent global snapshots. Unlike other systems, Retroscope does not need to block while taking snapshot, as it does not need to wait out time uncertainty caused by the clock skews at various machines thanks to the reliance on Hybrid Logical Clocks (HLC) instead of NTP-synchronized (NTP) time.
HLC introduces causality information into the clock as messages are being exchanged between servers and provides the same causal guarantees as Lamport’s Logical Clocks (LC). More information about HLC can be found here and here.
Taking a snapshot
Retroscope achieves snapshots by adding HLC into the network communication of the Retroscoped system. Internally, Retroscope keeps a sliding window-log of past state changes along with the associated HLC timestamps at each node. This window-logs are used to facilitate the unplanned nature of taking snapshots. Snapshots are triggered by a special client that maintains the common HLC with the rest of the system. Retroscope allows for instant unplanned snapshot to be started by the initiator, such instant snapshots are guaranteed to capture the states at the time Tnow of snapshot request being issued by the initiator. Obviously, once the snapshot request message reaches the nodes, the time has advanced to Tr > Tnow.
Upon receiving the snapshot request message at Tr, each node starts taking a local snapshot. Since the system does not halt processing requests, depending on the implementation, we may arrive to a local snapshot at some time Tf >= Tr. Because our local snapshot is at the state that happened after the requested time, we need to modify it to arrive to the state at time Tnow. We use the window-log of state changes to undo all operations that happened locally after Tnow, thus arriving to a desired local snapshot. Once all nodes compute local snapshots, Retroscope is done taking a global consistent snapshot at time Tnow.
Retroscope provides more flexibility in taking unplanned snapshots. Taking instant snapshot (i.e. snapshot initiated at Tnow) requires each node to maintain only a small log of recent changes. We can, however, expand the instant snapshots and offer retrospective snapshot flexibility at the expense of growing state change log larger. With retrospective snapshots, we can offer the ability to look at the state that has already happened in the past. This functionality is handy for application debugging when there is a need to investigate the root cause of the problem after it has already happened. A distributed reset is another application that can benefit from the retrospective snapshots, as the system can be reset into a correct state after the state has been corrupted.
We have Retroscoped Voldemort key-value database to take data-snapshots. Retroscoping Voldemort took less than a 1000 lines of code for adding HLC to the network protocol, recording changes in the Retroscope window-log, and performing snapshot on Voldemort’s storage. We did the experiments on the 10-node Voldemort cluster with databases of various sizes. We have learned that keeping the window-log of state changes has very little impact on the throughput and latency when no snapshots are being taken, as seen in figure below.
Performing snapshot is non-blocking in Retroscope, because there is no need to wait out the time uncertainty. The non-blocking nature allows Voldemort to continue processing both read and write requests while the snapshot is being computed. The figure below shows throughput and latency for every second of execution while taking the snapshot on a 10,000,000 items database (each item is 100 bytes). Overall we have observed an 18% throughput and 25% latency degradation over the snapshot time, however these numbers can be improved by using a separate disk system for snapshot.
What can be done with Retroscope?
We used Retroscope to take snapshots of data on a key-value store, however the utility of snapshots can be very extensive. With powerful snapshot capabilities, such as retrospective snapshot, we can look into the past of our distributed system and search for anomalies. Retrospective snapshots can be used to restore system to the latest correct state after the state corruption. Finding such correct states is also possible with Retroscope; we can use it to take successive snapshots to check for global invariants; it can be powerful in monitoring various application level predicates. Retroscope can perform other monitoring tasks. Unlike other monitoring systems that tend to look at local state independently or isolate monitoring to a request level, Retroscope can look at global parameters of the system across every node in a consistent way. We can even use Retroscope to detect erroneous patterns in message exchange by observing what messages are sent and received and how they impact the state at each node as we go through time.