The smallest large file part size, as indicated during authorization process by the server (in 2019 it used to be 5MB, but the server can set it dynamically)

account ID

An identifier of the B2 account (not login). Looks like this: 4ba5845d7aaf.

application key ID

Since every account ID can have multiple access keys associated with it, the keys need to be distinguished from each other. application key ID is an identifier of the access key. There are two types of keys: master application key and non-master application key.

application key

The secret associated with an application key ID, used to authenticate with the server. Looks like this: N2Zug0evLcHDlh_L0Z0AJhiGGdY or 0a1bce5ea463a7e4b090ef5bd6bd82b851928ab2c6 or K0014pbwo1zxcIVMnqSNTfWHReU/O3s

b2sdk version

Looks like this: v1.0.0 or 1.0.0 and makes version numbers meaningful. See Pinning versions for more details.

b2sdk interface version

Looks like this: v2 or b2sdk.v2 and makes maintaining backward compatibility much easier. See interface versions for more details.

master application key

This is the first key you have access to, it is available on the B2 web application. This key has all capabilities, access to all buckets, and has no file prefix restrictions or expiration. The application key ID of the master application key is equal to account ID.

non-master application key

A key which can have restricted capabilities, can only have access to a certain bucket or even to just part of it. See to learn more. Looks like this: 0014aa9865d6f0000000000b0


A container that holds files. You can think of buckets as the top-level folders in your B2 Cloud Storage account. There is no limit to the number of files in a bucket, but there is a limit of 100 buckets per account. See to learn more.