The new SatelliteCollections Feature of ArangoDB
With the new Version 3.2 we have introduced a new feature called SatelliteCollections. This post explains what this is all about, how it can help you, and explains a concrete use case for which it is essential.
Background and Overview
Join operations are very useful but can be troublesome in a distributed database. This is because quite often, a join operation has to bring together different pieces of your data that reside on different machines. This leads to cluster internal communication and can easily ruin query performance. As in many contexts nowadays, data locality is very important to avoid such headaches. There is no silver bullet, because there will be many cases in which one cannot do much to improve data locality.
One particular case in which one can achieve something, is if you need a join operation between a very large collection (sharded across your cluster) and a small one, because then one can afford to replicate the small collection to every server, and all join operations can be executed without network communications.
This is what the SatelliteCollections do. In ArangoDB, a SatelliteCollection is one that has exactly one shard which is automatically replicated synchronously to every DBserver in the cluster. Furthermore, the AQL query optimizer can recognize join operations with other sharded collections and organize the query execution such that it takes advantage of the replicated data and avoid a lot of costly network traffic.
The name “SatelliteCollection” comes from the mental image of a smallish collection (the satellite) which orbits around all the shards of a larger collection (the bigger planets), always close at hand for a next join operation, replacing interplanetary traffic with a quick surface orbit shuttle.
A concrete use case
Imagine that you have an IoT use case with lots of sensor data from a variety of devices. The set of individual events is clearly too large for a single server, but the number of devices is small enough to replicate to each server. If you now need to select certain events, usually restricted by their timestamp, but also filtered according to some condition on the device which produced the event, you have to bring together the data of an event with the data of its device.
One approach is to embed the device data with every event, thereby avoiding any join operation. This needs a lot more storage in the first place, but it also has other disadvantages. First of all, it contradicts the principle of data normalization, because device data is stored redundantly. Secondly (and of course related), there is no longer a single place holding all device data, so it is hard to get an overview over all devices or something like this.
The other sensible approach is to keep the device data separately, say in a smaller collection, and join an event to its device via a foreign key. This is the usual relational approach, which makes a lot of sense.
However, it requires a join operation for a query like this:
FOR e IN events
FILTER e.timestamp >= "123123123223" && e.timestamp <= "125212323232"
FOR d IN devices
FILTER e.device == d._key
FILTER d.firmwareVersion == "17"
Obviously, we assume that the large events collection has a sorted index on the
This fetches all events in a prescribed time range whose corresponding device has firmware version 17, which is a query that could very well occur in practice. There are lots of similar use cases, the crucial ingredient is that a join between a large and a small collection is needed.
Let’s see what the query engine has to do to execute the above query.
It almost certainly wants to use the index on `timestamp`, so, on each shard of the `events` collection it uses this index to find the corresponding events. For each such event found it now has to find the corresponding device to do the filtering on the `firmwareVersion` attribute.
In the non-satellite case the query engine has to take all found events, move them to the place where the small devices collection is held, then perform a lookup for each such event, and then either discard the event or put it into the result set, if the firmware version matches.
ArangoDB will even collect all results from all shards on the coordinator and then send them on to the DBserver that holds the devices. The result set will be sent back to the coordinator and then to the client.
Let’s analyze this with a few concrete numbers. Assume that there is billion events and a million of them lie in the time range in which we are interested. Assume furthermore, that about one in 1000 devices has the particular firmware and that the events are evenly distributed between the devices.
Then the above query will find a million out of a billion, distributed across the shards of the `events` collection. Altogether, it will send a million events across the network to the coordinator (each from one of the shards), and then send all of them to the DBserver holding the devices. There it will perform a million lookups locally and discard 999 out of 1000 devices, producing 1000 results, which are sent back to the coordinator. Altogether, we have sent a million events two times over the wire, this is expensive.
If the `devices` collection is a SatelliteCollection, then the data is resident on each server, that is, close to each shard of the `events` collection. Therefore, the query optimizer can select the events in the time range in question on each shard, and directly perform the lookup for the device, without any network traffic. It discards most of them because of the firmware version and only has to send the 1000 results back to the coordinator and then to the client. It could even perform this in a completely parallel fashion, but this is a future performance enhancement.
That is, the net savings effect, in this case, is not to have to send a million events across the wire twice. If you assume that an event uses 500 bytes, then this is network transfer of approximately 1GB. On a fast ethernet this takes approximately 100s. On gigabit ethernet it is only 8s under optimal conditions, but this is still noticeable.
Our experiments actually confirm these savings in network traffic and time. In the coming weeks we will publish a benchmark including all the data used and measurements done, so watch out for more details.
How to try it out
Obviously, a collection that is replicated synchronously to all DBservers has a larger latency on writes. However, we would assume that such a SatelliteCollection has a smaller write bandwidth than the larger events collection.
Let me close with another word of warning: SatelliteCollections are a nice and very useful feature, but they need planning to unfold their full potential. Please contact us, if you have a particular use-case in mind.
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