The Consumer

The ADS consumer works by issuing “fetch” requests to the brokers leading the partitions it wants to consume. The consumer specifies its offset in the log with each request and receives back a chunk of log beginning from that position. The consumer thus has significant control over this position and can rewind it to re-consume data if need be.

An initial question we considered is whether consumers should pull data from brokers or brokers should push data to the consumer. In this respect ADS follows a more traditional design, shared by most messaging systems, where data is pushed to the broker from the producer and pulled from the broker by the consumer. Some logging-centric systems, such as Scribe and Apache Flume, follow a very different push-based path where data is pushed downstream. There are pros and cons to both approaches. However, a push-based system has difficulty dealing with diverse consumers as the broker controls the rate at which data is transferred. The goal is generally for the consumer to be able to consume at the maximum possible rate; unfortunately, in a push system this means the consumer tends to be overwhelmed when its rate of consumption falls below the rate of production (a denial of service attack, in essence). A pull-based system has the nicer property that the consumer simply falls behind and catches up when it can. This can be mitigated with some kind of backoff protocol by which the consumer can indicate it is overwhelmed, but getting the rate of transfer to fully utilize (but never over-utilize) the consumer is trickier than it seems. Previous attempts at building systems in this fashion led us to go with a more traditional pull model.

Another advantage of a pull-based system is that it lends itself to aggressive batching of data sent to the consumer. A push-based system must choose to either send a request immediately or accumulate more data and then send it later without knowledge of whether the downstream consumer will be able to immediately process it. If tuned for low latency, this will result in sending a single message at a time only for the transfer to end up being buffered anyway, which is wasteful. A pull-based design fixes this as the consumer always pulls all available messages after its current position in the log (or up to some configurable max size). So one gets optimal batching without introducing unnecessary latency.

The deficiency of a naive pull-based system is that if the broker has no data the consumer may end up polling in a tight loop, effectively busy-waiting for data to arrive. To avoid this ADS has parameters in pull request that allow the consumer request to block in a “long poll” waiting until data arrives (and optionally waiting until a given number of bytes is available to ensure large transfer sizes).

Consumer Position

Keeping track of what has been consumed is, surprisingly, one of the key performance points of a messaging system.

Most messaging systems keep metadata about what messages have been consumed on the broker. That is, as a message is handed out to a consumer, the broker either records that fact locally immediately or it may wait for acknowledgement from the consumer. This is a fairly intuitive choice, and indeed for a single machine server it is not clear where else this state could go. Since the data structures used for storage in many messaging systems scale poorly, this is also a pragmatic choice – since the broker knows what is consumed it can immediately delete it, keeping the data size small.

What is perhaps not obvious is that getting the broker and consumer to come into agreement about what has been consumed is not a trivial problem. If the broker records a message as consumed immediately every time it is handed out over the network, then if the consumer fails to process the message (say because it crashes or the request times out or whatever) that message will be lost. To solve this problem, many messaging systems add an acknowledgement feature which means that messages are only marked as sent not consumed when they are sent; the broker waits for a specific acknowledgement from the consumer to record the message as consumed. This strategy fixes the problem of losing messages, but creates new problems. First of all, if the consumer processes the message but fails before it can send an acknowledgement then the message will be consumed twice. The second problem is around performance, now the broker must keep multiple states about every single message (first to lock it so it is not given out a second time, and then to mark it as permanently consumed so that it can be removed). Tricky problems must be dealt with, like what to do with messages that are sent but never acknowledged.

ADS handles this differently. Our topic is divided into a set of totally ordered partitions, each of which is consumed by exactly one consumer within each subscribing consumer group at any given time. This means that the position of a consumer in each partition is just a single integer, the offset of the next message to consume. This makes the state about what has been consumed very small, just one number for each partition. This state can be periodically checkpointed. This makes the equivalent of message acknowledgements very cheap.

There is a side benefit of this decision. A consumer can deliberately rewind back to an old offset and re-consume data. This violates the common contract of a queue, but turns out to be an essential feature for many consumers. For example, if the consumer code has a bug and is discovered after some messages are consumed, the consumer can re-consume those messages once the bug is fixed.

Offline Data Load

Scalable persistence allows for the possibility of consumers that only periodically consume such as batch data loads that periodically bulk-load data into an offline system such as Hadoop or a relational data warehouse.

In the case of Hadoop we parallelize the data load by splitting the load over individual map tasks, one for each node/topic/partition combination, allowing full parallelism in the loading. Hadoop provides the task management, and tasks which fail can restart without danger of duplicate data – they simply restart from their original position.

Consumer Offset Tracking

The high-level consumer tracks the maximum offset it has consumed in each partition and has the capability to commit offsets so that it can resume from those offsets in the event of a restart. ADS provides the option to store all the offsets for a given consumer group in a designated broker (for that group) called the group coordinator. i.e., any consumer instance in that consumer group should send its offset commits and fetches to that group coordinator (broker). For high-level consumers, this operation is performed automatically, while the simple consumer manages its offsets manually, since currently automatic transmission of offsets to simple consumers is not supported Java, they can only manually capture or extract offsets in ZooKeeper. When using Scala, a consumer can obviously commit or receive offsets in the coordinator.

When the group coordinator receives an OffsetCommitRequest, it appends the request to a special compacted ADS topic named __consumer_offsets. The broker sends a successful offset commit response to the consumer only after all the replicas of the offsets topic receive the offsets. In case the offsets fail to replicate within a configurable timeout, the offset commit will fail and the consumer may retry the commit after backing off. The brokers periodically compact the offsets topic since it only needs to maintain the most recent offset commit per partition. The coordinator also caches the offsets in an in-memory table in order to serve offset fetches quickly.

When the coordinator receives an offset fetch request, it simply returns the last committed offset vector from the offsets cache. In case coordinator was just started or if it just became the coordinator for a new set of consumer groups (by becoming a leader for a partition of the offsets topic), it may need to load the offsets topic partition into the cache. In this case, the offset fetch will fail with an CoordinatorLoadInProgressException and the consumer may retry the OffsetFetchRequest after backing off.

Migrating offsets from ZooKeeper to ADS

To migrate consumers and their offsets from ZooKeeper to ADS:

  1. Set offsets.storage=ads and dual.commit.enabled=true in your consumer config.
  2. Do a rolling bounce of your consumers and then verify that your consumers are healthy.
  3. Set dual.commit.enabled=false in your consumer config.
  4. Do a rolling bounce of your consumers and then verify that your consumers are healthy.

A roll-back (i.e., migrating from ADS back to ZooKeeper) can also be performed using the above steps if you set offsets.storage=zookeeper.