abacus-mongoclient
Subset of the MongoDB API over a set of DB partitions
Last updated 2 years ago by hsiliev .
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abacus-mongoclient

Distributes DB operations over a set of DB partitions with MongoDB backend.

Dependencies

The module is using mongodb native NodeJS driver.

Differences between Couch and Mongo

Revisions

Couch supports revisions of documents using the _rev field. Updates are done specifying the previous revision in the document. If _rev is not specified then a new documents is created. That's why Abacus modules should store the _rev field in memory, to do an update.

Mongo does not have this concept natively and mongoclient have to simulate it by simply storing the _rev field and switching between insert and update operation modes.

User provided vs stored fields

Documents in Couch have both _id, id, _rev and rev fields. The underscored ones are provided by the user, while the non-prefixed ones are the ones stored by CouchDB.

The API function callbacks with CouchDB client module will return the non-prefixed id and rev fields since they return the stored object metadata.

To keep the same behaviour the MongoDB client returns the id and rev fields.

Duplicated documents

The multi-document functions such as allDocs and bulkDocs return the result in the same order as the input array of document ids.

If the list of ids contains duplicated entries:

id1
id2
id3
id1

then the result with CouchDB is:

value1
value2
value3
value1

Mongo on the other hand does not return duplicating results due to the semantics of the used find. The client mimics the Couch client and builds in-memory the same result set, although the DB call returns just the first 3 values.

Document order

CouchDB returns the documents in the order the request ids are presented. Mongo maintains natural order by default, which means that the client has to reorder the results in the expected by the client form.

This is done in the same code as the deduping logic described above in the batchOp and bulkOp functions.

Range operations

CouchDB requires the start and end keys to be swapped in allDocs if the user selected descending order of the result.

Mongo client maintains the same requirement and internally swaps the keys before passing them to the underlying Mongo driver.

Operation metadata

The Couch database returns the status of the operation and the stored id and rev fields. Mongo returns a different result set for the different operations.

To comply with the couchclient API this module returns the same metadata, returning the input docs in case of successful operation completion. This is done to avoid additional round-trip to the database.

Error codes

CouchDB communicates over HTTP/HTTPS. This makes it easy to include HTTP response codes, such as 409 or 404 in its driver/client errors.

These codes are propagated to abacus-retry and abacus-breaker modules by setting noretry and nobreaker flags in the error.

This client adds the same flags, based on different criteria - error code 11000 for instance.

Collections

This module adds support for the collections in MongoDB. If the URI is in the form mongodb://user:password@host:port/database/collection, then the documents will be saved under the specified collection.

If no collection is specified, then the default collection documents will be used.

Partitioning

The partitioning in the Mongo client is based on the whole URI since it can contain database and collection, while in Couch client the partitioning is based on database name only.

The partition names are constructed by using the last component in the URI (database or collection) and appending partition suffix.

This enables the use of managed databases, that usually provide service instances with URI including collection.

URI

This client appends all options specified by the user to the partitions created during its normal functioning. An example is the Mongo schema, specified as option ?ssl=true after the address.

Connection options

Couch client passes skip_setup: true option to avoid creating a database on read access. In Mongo this is the default behaviour and we don't need any additional flags.

The Mongo client uses a pool of 1 connection since every DB is pooled internally. It also uses socket keep alive and connection timeout of 30000 ms by default.

Implementation

The mongoclient module implements the same API as couchclient. In this way it can serve as drop-in replacement and requires extremely thin abstraction layer, implemented in dbclient.

All functions exposed by the API open a collection and then translate the API to Mongo driver primitives.

get

Performs find with the specified _id. Returns the found document or undefined if not found (and not error!). The returned document has _rev set to 1 by default.

put

Performs:

  • update (with {upsert: true} option) if document with _rev is passed or
  • insertOne with the provided document

Returns the input document with id equal to _id and _rev set to 1 by default. Translates 11000 error code into 409 status.

remove

Calls deleteOne with the _id of the document. Returns the input document with id equal to _id and _rev with value from the stored document.

allDocs

Has two flavours based on the options passed. If startkey and endkey are present in the options calls the range variant. Otherwise uses the list one.

  • Range

    Uses find with:

    • { $gte: opt.startkey, $lte: opt.endkey }, or
    • { $gte: opt.endkey, $lte: opt.startkey } if descending: true
  • List

    Uses find with { $in: opt.keys }.

Both variants filter out the value field if include_docs: false. They return the found documents with id equal to _id and _rev set to 1 by default.

bulkDocs

Uses ordered bulk operation and:

  • adds in the bulk with upsert().updateOne operations for each document that has _rev
  • inserts with insert operation for document that has no specified _rev

Returns the input documents with id equal to _id and _rev set to 1 by default.

batch_get

Performs find with { $in: [ <id1>, <id2>, ... , <idN> ].

Filters out the value field if include_docs: false. Returns the found documents with id equal to _id and _rev set to 1 by default.

batch_put

Uses ordered bulk operation and:

  • adds in the bulk with upsert().updateOne operations for each document that has _rev
  • inserts with insert operation for document that has no specified _rev

Returns the input documents with id equal to _id and _rev set to 1 by default.

batch_remove

Uses deleteMany with all docs batched. Returns the found documents with id equal to _id and _rev set to 1 by default.

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