postgres-entities
Store JSON documents in Postgres
Last updated 2 years ago by jhford .
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NOTE: this library is under initial development

This is a library for storing objects with a defined structure and optionally attached behaviour

Design Goals

This library was written to replace the Azure Entities library which the Taskcluster platform was built upon. There were three major motivations for building this library: redeployability, visibility into data and an increasingly unreliable Azure service.

Similar interface to Azure Entities

The Azure Entities library has a very nice interface for being able to build services which are reliable and scalable by default. This library is designed to take the concepts of the Azure Entities library and apply them to a library backed by Postgres instead.

Reliable ETag and Last-Modified values

When used in a backend server, there's a very strong chance that each entity represents an API entity. For HTTP based services, having reliable etag and last-modified headers enables more advanced request handling. Since these values are both reflections of the data being stored in the database, it would be ideal if the persistence layer could automatically track when changes are made to the underlying data.

This library accomplishes this by having two columns for the etag and last modified. Both are set as the default value on the table for initial insertion and all managed tables have a stored procedure that is triggered on updates which creates new values if and only if the values are changed.

Zero downtime service upgrades

One of the nice features of Azure Entities is the ability to perform zero downtime upgrades. This was accomplished by having a firm storage format in the data storage system and a user definable format for entries. A user of the library can define the structure of the documents they want to store by describing the fields, marking some as being part of the unique identifier for that document. Each version of the document contains a version number and a function which can do a migration from the previous version.

The rough flow of an upgrade for this library would be:

  1. Deploy services with newest version of the entity declaration
  2. Once all services have newest version, initiate a migration for all entities stored
  3. Once migration is completed, the code for older versions can safely be removed from the service if desired

During the time between the service deployment beginning and being completed it is possible that an old service instance tries to read an entity from a newer version. In this case, the library will refuse to load the entity by throwing an error. Since services should be implemented in a way where requests are safe to retry, the next attempt by the client to perform that change will hopefully be picked up by a newer instance of the service.

Understandable Data Format

Azure Entities is unfortunately very complicated for other consumers to understand. Looking at the data without using Azure Entities was basically impossible. Without having the actual azure credentials, it is also impossible to run queries directly on the dataset. Even with those credentials, no tools could understand the data stored within.

A goal of this library is to have a data storage format that a human can understand just looking at it. This enables easier debugging as well as enabling others to develop tools which can perform queries on the data stored in the database. The data format used by this library is explained later in this document.

The goal here isn't to enable the database as an interchange format, but rather allow scripts to be written which examine the queue. This would ideally be done using a read-only clone of the database.

Reliable Unit Tests without Database Configuration

A major goal of this library is to enable users to write unit tests which require absolutely zero database configuration. This library contains all the tooling required to set up a local database that is specifically for running tests against, managing that process as well as showing logs. It is also design to recreate the database from scratch for each test.

Furthermore, the entities are able to generate a complete set of queries which can create every needed Postgres object (e.g. Tables, Triggers, Indexes, Extensions, etc) to run the code against. This means that it is not possible to forget which database objects need to be created, since the tests won't pass without them. It also means that each test runs in a clean database to avoid intermittent test failures.

The only dependency on the system besides a supported Node.js environment is that the system have a working postmaster and initdb command. Optionally a psql local command can be used to run sql files and commands through psql. Managing the postmaster process and setting up the database directory with initdb (PGDATA) is completely managed by the test harness provided. By setting the POSTGRES_LOG_STDIO variable to any value, the Postgres log file can be written to stanard output with verbose logging to show the actual queries being run by Postgres Entities.

In addition to the benefits for tests being more reliable, it also ensures that the database tests can be run without having any impact on the system installation of Postgres. When testing in CI environments, root is not required for anything other than ensuring Postgres is itself installed.

These tools are exported for use by services built with this library to have automatic database management in unit tests.

In order to ensure that the system's database is within acceptable version numebers, these tools support optionally defining a maximum and minimum supported version of Postgres. For example, to use the ON CONFLICT support in the INSERT command, you must have Postgres 9.5 or higher. In this case, the maximum can be specified to this library. If the tests are attempted on a Postgres 9.4 system, an error will be raised.

Optimistic Concurrency

Reducing the amount of time that locks are held improves overall system performance in many cases. This library as well as Azure Entities do all edits of documents using an optimistic concurrency model. Users will load a document, perform the changes required and then update the document in the database if and only if the database's version of the etag matches that of the document on which the modifications were changed.

Ths simple approach to building this library would have been to edit each document in a SQL transaction. The downside to this approach is that a row lock is held while the entire transaction is run. If the operation to change the document takes a very long time, it could cause delays in other request.

In this library, the user will load a copy of the document from the database and hold absolutely no locks. In fact, this load can be executed against a read only copy of the database. The user can then do whatever is needed to modify the document. Only when the document is finished being edited, Postgres Entities will attempt to modify the data. In order to safely do the update only if the etag has not changed, a small transaction is done to read the etag in and check if it's changed. This data persistence does hold a row lock for a short time for inserting into the database.

In the case that there is a collision (i.e. something else modified the data) an identifiable error is raised and the service can reload the object and try again.

No SQL commands in services

Users of this library should not need to write any SQL commands to accomplish any of the supported operations.

Database Deployment Options

This library supports running each service in a seperate database as well as running multiple services in a single database. For users of this library, each service sees the same view of the database whether it is the only service in the database or not.

There's also support for read-only mirror databases for operations which that is supported. By default, all operations are done against a read/write connection. If so configured, the database can direct operations which only involve a view of the data to a read only copy.

TODO Searching

This is not yet implemented, but this library is designed to make it possible to search for documents based on strict conditions and stream results.

TODO Queueing

Azure Entities did not have support internally for building queues. In order to build queues, the Azure Queue Storage system was instead used. The drawback to this queue is that there is zero visibilty into the contents of the queue while it's in operation and it can only be modified by pushing and popping. This library will support building work queues of documents inside of it.

This will allow more advanced queueing systems to be build as well as the ability to query the queue for information about queued items.

Data Storage Format

The database table format is designed to be static, with the user facing contents being able to migrate between different definitions as required.

There are a few different levels of storage in this library and it's important to understand the difference between each one to understand the internal operation of the library

Document JS-Values

Documents are JS objects with a few special properties:

  1. Each property defined directly on the object is considered to be a 'Field'.
  2. Each field has special behaviour for converting to the database
  3. The constructor of the Document must take exactly an object of values for the fields and set those as own properties
  4. Each object must not alter the .__etag, .__fields or .__version properties

This level defines the values which services will operate on.

Ids

Each Entity can define specific properties which make up a part of the ID for the object in the database. It is a list of field names, and the system will determine the key of each document based on values of the document. These ids are duplicated from the document into the database column because the PRIMARY KEY constraint of a table does not support being applied to members of a JSON column.

Where needed to be specified, IDs must always be given in Document JS-Values. When there is a single field which is the ID field for that entity, a string can be used as short hand. Otherwise any object which has properties named the ID fields with valid values, including a document, can form the id as passesd around

Take a hypothetical PGEntity instance which has an id value of ['workerGroup', 'workerId']. A valid key would be {workerGroup: 'us-east-1', workerId: 'i-abcdef1234567'}. These keys can be over specified to allow passing an entire document to derive the key from.

In another example where the id value is ['taskId'], a valid key could be 'abced1234' as a shorthand.

Raw JS-Values

Between the Document JS-Values and those values passed through serialised JSON to Postgres are JS-Values which represent the values which will be JSON serialised. Each type of field understands how to convert from database format using its fromDB method and into database format using its toDB method.

An example is for the BigInt type. In Document JS-Values, this is represented as an actual BigInt variable. In Raw JS-Values, it is represented as a hexidecimal string (e.g. '0xDEADBEEF') so that it can be accurately be serialised to JSON for storage.

When serialising, properties defined on documents which aren't fields will be ignored. This is so that any behaviours associated with the document can add intermediate data as properties on the document.

Postgres Values

The Postgres table has the following keys:

  1. id TEXT: the id of the document. It is duplicated from the document's values because of restrictions of the PRIMARY KEY constraint in Postgres. The value stored here is the serialised format.
  2. value JSONB: a json document which has the complete contents of the document. This document is a json document where each top level property is one of the defined fields.
  3. touched TIMESTAMPTZ: a timestamp of the last time this document was modified. This value is automatically managed and should not be edited.
  4. etag UUID: a uuid value for this document. This value is automatically managed and should not be edited. It is a v4 uuid generated by the pgcrypto library, which internally uses OpenSSL.
  5. sequence BIGINT: a number which represents the sequence in which the document was inserted initially into the database. This is primarily used for internal purposes, specifically pagination

There is a dependency on the pgcrypto Postgres extension. This extension is currently provided by Amazon RDS, Google Cloud SQL, Azure Postgres, Heroku Postgres, Fedora Postgres and Debian Postgres.

ID Serialisation Format

The ID used in the database is the base64 encoded string value of the fields from the document. The order of fields specified in the PGEntity definition is the order of the fields in the id.

This code will generate a valid database id column value for a PGEntity which uses an id value of ['name', 'vorname']:

`${Buffer.from('Ford').toString('base64')}_${Buffer.from('John').toString('base64')}`

This format is used so that the simplist of a single field key has a simple mapping to database id, while still supporting composite keys.

Migration

Each migration is performed by a function which receives a copy of the field values of a document. It does not receive an actual document itself. This is to ensure that when a document has behaviour attached to it (e.g. methods) those methods only need to be defined for the latest version. Migration functions should be removed once there's no longer a chance of them being found in the wild.

Migration occurs on all loads of documents which are at a version lower than the current version. It only occurs on loads, and loading does not persist the migration. This is to support read-only databases. Since the migration is done on every load, all update operations will result in the document being stored with the new version.

Examples

As is often the case, the unit tests for this library serve as the best examples for usage. Some specific examples are provided here to illustrate the library.

Imports

This the maximum extent of imported constructors to use this library

const {
  PGEntityError,
  PGEntityManager,
  PGEntity,
  PGEntityDocument,
  PGStringField,
  PGIntegerField,
  PGBigIntField,
  PGDateField,
} = require('postgres-entities');

If you're building a service using taskcluster-lib-loader, the PGEntityManager should be instantiated once for the whole service, one Entity for type to be persisted and any custom DocumentConstructor classes should be built as code outside of the loader file.

Define an Entity

let manager = new PGEntityManager({
  service: 'phonebook',
  connectionString: process.env.DATABASE_URL,
});

let taskEntity = new PGEntity({
  name: 'task',
  id: ['taskId'],
  manager,
  versions: [{
    fields: {
      taskId: PGStringField,
      command: PGStringField,
      priority: PGIntegerField,
    },
    indexes: ['priority'],
  }],
});

Define an Entity with a custom Document type

If so desired, a different constructor can be given to the PGEntity constructor.

This is useful when custom behaviour is desired.


class Task extends PGEntityDocument {
  priorityFactor(x) {
    return this.priority * x;
  }
}

let taskEntity = new PGEntity({
  name: 'task',
  id: ['taskId'],
  manager,
  DocumentConstructor: Task,
  versions: [{
    fields: {
      taskId: PGStringField,
      command: PGStringField,
      priority: PGIntegerField,
    },
    indexes: ['priority'],
  }],
});

Create, Insert, View, Update and Remove an Entity

let document = taskEntity.createDocument({
  value: {
    taskId: slugid.nice(),
    command: 'echo hello, world',
    priority: 10,
});

await taskEntity.insert(document);

await taskEntity.update(document, () => {
  document.priority = 20;
});

await taskEntity.remove(document);

Acknowledgments

Jonas built the library that served as inspriation for a lot of the design. This library has served the Taskcluster team very well for nearly 5 years and has been key to the ability to rapidly build and iterate on services.

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