API in cluster mode

There is a cluster of several servers that synchronize data between each other. Client should always work with the same server to ensure consistency between separate requests to the CDB. That is why cookie is required while sending POST/PUT/PATCH/DELETE requests. Cookies provide server stickiness. You can get such cookie via GET request and then use it for POST/PUT/PATCH/DELETE.

If during operations the server requested by cookie went down or is unavailable, client will receive status code 412 of request and new cookie to use. Request should be repeated with new cookie.

Warning

Since we moved to MongoDB cluster with primary & secondaries, SERVER_ID cookie is no longer required.

Please read the new instructions below

Causal consistency sessions

MongoDB clients with causal consistency sessions can achieve the following guarantees:

Read own writes
Monotonic reads
Monotonic writes
Writes follow reads

Since our clients work with database through API, they will need to store their session state. So that API is able to apply their parameters and provide the guaranties to every user.

_images/mongodb.png — Fig. 1. Database connection configuration overview diagram

_images/session.png — Fig. 2. Consistent database session mechanism sequence diagram

Here is an example

Every request returns a session cookie, that is continuously updated

POST /tenders/64e93250be76435397e8c992ed4214d1/bids HTTP/1.1

Content-Type: application/json
{
    "data": {
        ...
    }
}


HTTP/1.1 200 Created
Content-Type: application/json
Set-Cookie: SESSION=0KjQvtCxINGI0L4/IA==; Path=/
{
    "data": {
        "id": "ddd45992f1c545b9b03302205962265b",
        "status": "draft",
        ...
    }
}

Then the following request should use that cookies

POST /tenders/64e93250be76435397e8c992ed4214d1/bids HTTP/1.1

Content-Type: application/json
Cookie: SESSION=0KjQvtCxINGI0L4/IA==
{
    "data": {
        ...
    }
}


HTTP/1.1 200 Created
Content-Type: application/json
Set-Cookie: SESSION=0JHQsNC90LTQtdGA0L7QvNC+0LHRltC70ZY=; Path=/
{
    "data": {
        "status": "active",
    }
}