Google Cloud Platform Blog: Building immutable entities into Google Cloud Datastore

Building immutable entities into Google Cloud Datastore

Wednesday, August 3, 2016

Posted by Aleem Mawani, Co-Founder, Streak.comEditor's note: Today, we hear from Aleem Mawani, co-founder of Streak.com, a Google Cloud Platform customer whose customer relationship management (CRM) for Google Apps is built entirely on top of Google products: Gmail, Google App Engine and Google Cloud Datastore. Read on to learn how Streak added advanced functionality to the Cloud Datastore object storage systemGoogle Cloud PlatformGoogle App EngineGoogle Cloud Datastore

We wanted an easy way to implement a newsfeed-style UI. Typical newsfeeds show how an entity has changed over time in a graphical format to users. Traditionally we stored separate side entities to record the deltas between different versions of a single entity. Then we’d query for those side entities to render a newsfeed. Designing these side entities was error prone and not easily maintainable. For example, if you added a new property to your entity, you would need to remember to also add that to the side entities. And if you forgot to add certain data to the side entities, there was no way to reconstruct that later down the line when you did need it — the data was gone forever.

The "Contact" entity stores data about users’ contacts. Because it’s implemented as an immutable entity, it's easy to generate a historical record of how that contact has changed over time.

Having immutable entities allows us to recover from user errors very easily. Users can rollback their data to earlier versions or even recover data they may have accidentally deleted (see how we implemented deletion below)¹.

Potentially easier debugging. It’s often useful to see how an entity changed over time and got into its current state. We can also run historical queries on the number of changes to an entity - useful for user behaviour analysis or performance optimization.

Some context—denormalize

Queries that are restricted to a single entity group get consistent results. This means that a write immediately followed by a query will have results that are guaranteed to reflect the changes made by the write. Conversely, if your query is not limited to a single entity group you may not get consistent results (stale data).

Multi-entity transactions can only be applied within a single entity group (this was recently improved — Cloud Datastore now supports cross entity group transactions but limits the number of entity groups involved to 25).

documentation How we implemented immutable entities—

(click to enlarge)

isTipversionIdconsistentId—firstEntityInChainObjectify

public class ImmutableDatastoreEntity {

@Id

Long versionId;

@Parent

Key<T> firstEntityInChain;

protected Long consistentId;

protected boolean isTip;

Key<User> savedByUser;

}

Performing createsversionIdconsistentIdfirstEntityInChainisTip

ImmutableDatastoreEntity entity = new ImmutableDatastoreEntity();

entity.setVersionId(DAO.allocateId(this.getClass()));

entity.setConsistentId(entity.getVersionId());

entity.setFirstEntityInChain((Key<T>) Key.create(entity.getClass(), entity.versionId));

entity.setTip(true);
Performing updates To update a logical entity with new data, we first need to fetch the most recent datastore entity in the chain (we describe how in the "get" section below). We then create a new datastore entity and set the consistentId and firstEntityInChain to that of the previous datastore entity in the chain. We set isTip to true on the new datastore entity and set it to false on the old datastore entity (note this is the only instance in which we modify an existing entity so we aren’t 100% immutable).
We finally fill in the timestamp and user keys fields, and we’re ready to store the new datastore entity. Two important points on this: for the new datastore entity, we can let the datastore automatically allocate the ID when storing the entity (because we don’t need to use it anywhere else). Second, it's incredibly important that we fetch the existing datastore entity and store both the new and old datastore entity in the same transaction. Without this, our data could become internally inconsistent.

// start transaction

ImmutableDatastoreEntity oldVersion = getImmutableEntity(immutableId)

oldVersion.setTip(false);

ImmutableDatastoreEntity newVersion = oldVersion.clone();

// make the user edits needed

newVersion.setVersionId(null);

newVersion.setConsistentId(this.getConsistentId());

newVersion.setFirstEntityInChain(oldVersion.getFirstEntityInChain());

// .clone also performs the last two lines but just to be explicit this, just fyi

newVersion.setTip(true);

ofy().save(oldVersion, newVersion).now();

// end transaction

Performing gets Performing a get actually requires us to do a query operation to the datastore because we need to find the datastore entity that has a certain consistentId AND has isTip set to true. This entity will represent the logical entity. Because we want the query to be consistent, we must perform an ancestor query (i.e., tell Cloud Datastore to limit the query to a certain entity group). This only works because we ensured that all datastore entities for a particular logical entity are part of the same entity group.

This query should only ever return one result —

Key ancestorKey = KeyFactory.createKey(ImmutableDatastoreEntity.class, consistentId);

ImmutableDatastoreEntity e = ofy().load()

.kind(ImmutableDatastoreEntity.class)

.filter("consistentId", consistentId)

.filter("isTip", true)

.ancestor(ancestorKey) // this limits our query to just the 1 entity group

.list()

.first(); Performing deletes In order to delete logical entities, all we need to do is set the isTip of the most recent datastore entity to false. By doing this we ensure that the "get" operation described above no longer returns a result, and similarly, queries such as those described below continue to operate.

// wrap block in transaction

ImmutableDatastoreEntity oldVersion = getImmutableEntity(immutableId);

oldVersion.setTip(false);

ofy().save(oldVersion, newVersion).now(); Performing queries We need to be able to perform queries across all logical entities. However, when querying every datastore entity, we need to modify our queries so that they only consider the tip datastore entity of each logical entity (unless you explicitly want to find old versions of the data). To do this, we need to add an extra filter to our queries to just consider tip entities. One important thing to note is that we cannot do consistent queries in this case because we cannot guarantee that all the results will be in the same entity group (in fact we know for certain they are not if there are multiple results)

List<ImmutableDatastoreEntity> results = ofy().load()

.kind(ImmutableDatastoreEntity.class)

.filter("isTip", true)

.filter(/** apply other filters here */)

.list(); Performing newsfeed queries One of our goals was to be able to show how a logical entity has changed over time, so we must be able to query for all datastore entities in a chain. Again, this is a fairly straightforward query —consistentId

Key ancestorKey = KeyFactory.createKey(ImmutableDatastoreEntity.class, consistentId);

List<ImmutableDatastoreEntity> versions = ofy().load()

.kind(ImmutableDatastoreEntity.class)

.filter("consistentId", consistentId)

.ancestor(ancestorKey)

.list(); Downsides Using the design described above, we were able to achieve our goal of having roughly immutable entities that are easy to debug and make it easy to build newsfeed-like features. However, there are some drawbacks to this method:

We need to do a query any time we need to get an entity. In order to get a specific logical entity, we actually need to perform a query as described above. On Cloud Datastore, this is a slower operation than a traditional "get" by key. Additionally, Objectify offers built-in caching, which also can’t be used when trying to get one of our immutable entities (because Objectify can’t cache queries). To address this, we’ll need to implement our own caching in memcache if performance becomes an issue.

There's no method to do a batch get of entities. Because each query must be restricted to a single entity group for consistency, we can’t fetch the tip datastore entity for multiple logical entities with just one datastore operation. To address this, we perform multiple asynchronous queries and wait for all to finish. This isn’t ideal or clean, but it works fairly well in practice. Remember that on App Engine there's a limit of 30 outstanding RPCs when making concurrent RPC calls, so this only takes you so far.

High implementation cost for the first entity. We abstracted most of the design described above so that future immutable entities would be cheap for us to implement, however, the first entity wasn’t trivial to implement. It took us some time to iron out all the kinks, so it’s definitely only worth doing this if you very much need immutability or if you’ll be spreading the implementation cost across many use cases.

Entities are never actually deleted. By design, we don’t delete immutable entities. However, from a user perspective, they may have the expectation that once they delete something in our app, we actually delete the data. This also might be the expectation in some regulated industries (i.e., healthcare). For our use case, it wasn't necessary, but you may want to develop a system that maps over your dataset and finds fully deleted logical entities and deletes all of the datastore entities representing them in some batch task periodically.

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1This is very similar to the idea of MVCC (https://en.wikipedia.org/wiki/Multiversion_concurrency_control) which is how many modern databases implement transactions and rollback.

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