Colossus

ELI5: High-performance ephemeral ticket store (i.e. pre-DB tickets)

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This is Colossus a service for serving tickets at mass volume.

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Introduction

Colossus aims to coordinate between different services (mongodb, redis and internal memory) to provide a reasonable ticketing service. The design goal from the outset of Colossus is to be agnostic as possible about the payload of the data.

Colossus implements different storage types, strategies depending on the service it's utilizing. For example the counter service implements a time-series event storage via a LWW-element-set CRDT with limited inline garbage collection.

At a high level the counter service maintains sets of values, with each set ordered accordingly with an associated value timestamp. Similarly the store service is implemented as a LWW-element-set, but does not fully embody a CRDT completely because it doesn't fully abide by the following laws that the counter does.

CRDTs (conflict-free replicated data types) are data types on which the same set of operations yields the same outcome when performed regardless of order of the executions and duplications of the operations. This allows data convergence without the need for consensus between replicas. This allows for easier an implementation, because no consensus protocol implementation is required.

Operations on CRDTs need to adhere to the following laws:

• Associativity

  The grouping of operations don't matter.

  a + (b + c) = (a + b) + c

• Commutativity

  The order of the operations don't matter.

  a + b = b + a

• Idempotence

  Duplication of operations don't matter.

  a + a = a

Colossus implements a set data type, specifically the Last Writer Wins element set (LWW-element-set). The description of the LWW-element-set simply follows:

• An element is in the set, if its most-recent operation was an add.
• An element is not in the set, if it's most-recent operation was a remove.

A more formal description of a LWW-element-set, as informed by Shapiro, is as follows:

• Set S is represented by two internal sets, the add set A and the remove set R.
• To add an element e to the set S, add a tuple t with the element and the current timestamp t = (e, now()) to A.
• To remove an element from the set S, add a tuple t with the element and the current timestamp t = (e, now()) to R.
• To check if an element e is in the set S, check if it is in the add set A and not in the remove set R with a higher timestamp.

Colossus implements the above definition, but extends it by applying a basic asynchronous garbage collection. All nodes carry a timestamp which are then filtered out of the nodes if the timestamp has become expired.

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Servers

1. HTTP Server
2. Walker Server

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Replication

Colossus replicates data over multiple non-communicating clusters. A typical replication factor is 3. Colossus has two methods of replicating data:

1. During a write.
2. During a read-repair.

A write (insert or delete) is sent to all clusters. The overall operation returns success when the quorum is reached. Unsuccessful clusters might have been affected by a network partition (slow, failed, crash) and in case of a unsuccessful write then a read-repair might be triggered on a later read.

A read (select) is dependent on the read strategy employed. If the strategy queries several clusters, it might be able to spot a disjointment in the resulting sets. If so, the union set is returned to the client and in the background, a read-repair is triggered which lazily converges the sets across all the replicas.

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Fault tolerance

Colossus runs as a homogenous distributed system. Each Colossus instance can serve all requests (insert, delete, select) for a client, and communicates with all Redis instances.

A Colossus instance is effectively stateless, but holds transient state. If a Colossus instance crashes, three types of state are lost:

1. Current client connections are lost. Clients can reconnect to another Colossus instance and re-execute their operations.
2. Lost client connections can lead to unfulfilled stored requests, where different stores can hold inconsistent states. Rolling back and garbage collection should clean these states up, but there isn't 100% guarantee at this state.
3. Unresolved read-repairs are lost. The read repair might be triggered again during another read.

Since all store operations are idempotent, failure modes should not impede on convergence of the data.

Persistence is delegated to MongoDB (others to follow).

If a Redis instance is permanently lost and has to be replaces with a fresh instance, there are two options:

1. Replace it with an empty instance. Keys will be replicated to it via a read-repair. Aa more and more keys are replicated, the read-repair load will decrease and the instance will work normally. This process might result in data loss over the lifetime of a system. If the other replicas are also lost, non-replicated keys (keys that have not been requested and thus did not trigger a read-repair) are lost.
2. Replace it with a cloned replica. There will be a gap between the time of the last write respected by the replica and the first write respected by the new instance. The gap might be fixed by subsequent read-repairs.

Both processes can be expedited via a keyspace walker process. Nevertheless, these properties and procedures warrant careful consideration.

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Structure

The structure of Colossus sets out to be tunable depending on the work undertaken (currently not possible without a restart). It is possible to change the various strategies for each service so that a different approach can be utilized (performance vs memory vs bandwidth).

1. Coordinator

The coordinator's job is to work as a intermediary for all the various services, similar in principale to conventional controller, but the difference is that the coordinator has more of a role in managing the stores. If the coordinator encounters an error it should either attempt to recover (by repairing or rolling back) or fallback to another service when possible.

2. Farms

A farm is a collection of clusters (see below) that allow the creation of fail overs or improvement of speed with the aid of more servers. The Coordinator generally talks to Farms directly.

3. Clusters

A collection of services in a group, most of which are just pools of connections. The cluster will do the main calls to the services (mongodb, redis and etc) and can be tweaked to use normal serial commands or pipelined commands to reduce the network latency.

4. Pools

Pools hold a collection of connections directly to the service. To aid the performance of each service, multiple connections are created to help improve any latency issues whilst waiting for a request to come back, with the added benefit of not exhausting the network stack of connections.

5. Instrumentation

Through out the application as much instrumentation has been put in to the application to ensure that we can see if any performance/defects occur. This can be seen as either plaintext (files, stdout), statsd and etc.

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Naming

1. Colossus Computer

Colossus was the name of a series of computers developed by British codebreakers in 1943-1945 to help in the cryptanalysis of the Lorenz cipher. Colossus used thermionic valves (vacuum tubes) and thyratrons to perform Boolean and counting operations. Colossus is thus regarded[1] as the world's first programmable, electronic, digital computer, although it was programmed by plugs and switches and not by a stored program.

2. I am Colossus