amazon web services - How to use auto increment for primary key id in dynamodb

Question

I am new to dynamodb. I want to auto increment id value when I use putitem with dynamodb.

Is possible to do that?

Answer 1

This is anti-pattern in DynamoDB which is build to scale across many partitions/shards/servers. DynamoDB does not support auto-increment primary keys due to scaling limitations and cannot be guaranteed across multiple servers.

Better option is to assemble primary key from multiple indices. Primary key can be up to 2048 bytes. There are few options:

1. Use UUID as your key - possibly time based UUID which makes it unique, evenly distributed and carries time value
2. Use randomly generated number or timestamp + random (possibly bit-shifting) like: ts << 12 + random_number
3. Use another service or DynamoDB itself to generate incremental unique id (requires extra call)

Following code will auto-increment counter in DynamoDB and then you can use it as primary key.

var documentClient = new AWS.DynamoDB.DocumentClient();
var params = {
  TableName: 'sampletable',
  Key: { HashKey : 'counters' },
  UpdateExpression: 'ADD #a :x',
  ExpressionAttributeNames: {'#a' : "counter_field"},
  ExpressionAttributeValues: {':x' : 1},
  ReturnValues: "UPDATED_NEW" // ensures you get value back
};
documentClient.update(params, function(err, data) {});
// once you get new value, use it as your primary key

My personal favorite is using timestamp + random inspired by Instagram's Sharding ID generation at http://instagram-engineering.tumblr.com/post/10853187575/sharding-ids-at-instagram

Following function will generate id for a specific shard (provided as parameter). This way you can have unique key, which is assembled from timestamp, shard no. and some randomness (0-512).

var CUSTOMEPOCH = 1300000000000; // artificial epoch
function generateRowId(shardId /* range 0-64 for shard/slot */) {
  var ts = new Date().getTime() - CUSTOMEPOCH; // limit to recent
  var randid = Math.floor(Math.random() * 512);
  ts = (ts * 64);   // bit-shift << 6
  ts = ts + shardId;
  return (ts * 512) + randid;
}
var newPrimaryHashKey = "obj_name:" + generateRowId(4);
// output is: "obj_name:8055517407349240"