Working with Different Data Centers

The servers are divided into several data centers (hereinafter “DCs”) in different parts of the world.
A complete list of proxy access points for these DCs may be obtained using help.getConfig:

dcOption#18b7a10d flags:# ipv6:flags.0?true media_only:flags.1?true tcpo_only:flags.2?true cdn:flags.3?true static:flags.4?true id:int ip_address:string port:int secret:flags.10?bytes = DcOption;
config#330b4067 flags:# phonecalls_enabled:flags.1?true default_p2p_contacts:flags.3?true preload_featured_stickers:flags.4?true ignore_phone_entities:flags.5?true revoke_pm_inbox:flags.6?true blocked_mode:flags.8?true pfs_enabled:flags.13?true date:int expires:int test_mode:Bool this_dc:int dc_options:Vector<DcOption> dc_txt_domain_name:string chat_size_max:int megagroup_size_max:int forwarded_count_max:int online_update_period_ms:int offline_blur_timeout_ms:int offline_idle_timeout_ms:int online_cloud_timeout_ms:int notify_cloud_delay_ms:int notify_default_delay_ms:int push_chat_period_ms:int push_chat_limit:int saved_gifs_limit:int edit_time_limit:int revoke_time_limit:int revoke_pm_time_limit:int rating_e_decay:int stickers_recent_limit:int stickers_faved_limit:int channels_read_media_period:int tmp_sessions:flags.0?int pinned_dialogs_count_max:int pinned_infolder_count_max:int call_receive_timeout_ms:int call_ring_timeout_ms:int call_connect_timeout_ms:int call_packet_timeout_ms:int me_url_prefix:string autoupdate_url_prefix:flags.7?string gif_search_username:flags.9?string venue_search_username:flags.10?string img_search_username:flags.11?string static_maps_provider:flags.12?string caption_length_max:int message_length_max:int webfile_dc_id:int suggested_lang_code:flags.2?string lang_pack_version:flags.2?int base_lang_pack_version:flags.2?int = Config;
---functions---
help.getConfig#c4f9186b = Config;

In this context, this_dc is the number of the current DC, dc_options is a list of all DCs available at the moment, each of which has an id, ip, and port for establishing a connection. Please note that ip and port may change frequently, based on proxy server load and the user's current location.
Typically, each DC has at least one IPv4 and one IPv6 endpoint available.

To optimize client communication with the API, each client must use the connection to the closest access point for its main queries (sending messages, getting contacts, etc.). Therefore, knowing how to select a DC is required before communicating with the API.

Registration/Authorization

The auth.sendCode method is the basic entry point when registering a new user or authorizing an existing user. 95% of all redirection cases to a different DC will occur when invoking this method.

The client does not yet know which DC it will be associated with; therefore, it establishes an encrypted connection to a random address and sends its query to that address.
Having received a phone_number from a client, we can find out whether or not it is registered in the system. If it is, then, if necessary, instead of sending a text message, we request that it establish a connection with a different DC first (PHONE_MIGRATE_X error).
If we do not yet have a user with this number, we examine its IP-address. We can use it to identify the closest DC. Again, if necessary, we redirect the user to a different DC (NETWORK_MIGRATE_X error).

Testing Redirects

There are reserved phone number prefixes to test the correctness of the application’s handling of redirects between DCs. Read more in User Authorization article.

File Access

A file saved by a user with upload.saveFilePart will be available for direct download only from the DC where the query was executed. That is why each file has a dc_id parameter:

document#1e87342b flags:# id:long access_hash:long file_reference:bytes date:int mime_type:string size:int thumbs:flags.0?Vector<PhotoSize> video_thumbs:flags.1?Vector<VideoSize> dc_id:int attributes:Vector<DocumentAttribute> = Document;
photo#fb197a65 flags:# has_stickers:flags.0?true id:long access_hash:long file_reference:bytes date:int sizes:Vector<PhotoSize> video_sizes:flags.1?Vector<VideoSize> dc_id:int = Photo;

encryptedFile#4a70994c id:long access_hash:long size:int dc_id:int key_fingerprint:int = EncryptedFile;

userProfilePhoto#82d1f706 flags:# has_video:flags.0?true photo_id:long stripped_thumb:flags.1?bytes dc_id:int = UserProfilePhoto;
chatPhoto#1c6e1c11 flags:# has_video:flags.0?true photo_id:long stripped_thumb:flags.1?bytes dc_id:int = ChatPhoto;

To download the file, an encrypted connection to DC dc_id must be established and used to execute the upload.getFile query.
If an attempt is made to download the file over a wrong connection, the FILE_MIGRATE_X error will be returned.

Please note that encryption keys are not copied between DCs; therefore, the process of establishing an encrypted connection is started from the very beginning for each new DC. An issued auth_key can be associated with the current authorized user by using an authorization transfer.

User Migration

During the process of working with the API, user information is accumulated in the DC with which the user is associated. This is the reason a user cannot be associated with a different DC by means of the client. However, in the future, during prolonged communication from an unusual location, we may decide that the user’s data must be moved to a different DC. After some time, the data will be copied and the association will be updated. Once this happens, when executing any query transmitted to the old DC, the API will return the USER_MIGRATE_X error. The client will then have to establish a connection with the new DC and repeat the query.

Authorization Transfer

The following methods can be used to eliminate the need for users to enter the code from a text message every time:

auth.exportedAuthorization#b434e2b8 id:long bytes:bytes = auth.ExportedAuthorization;
auth.authorization#cd050916 flags:# tmp_sessions:flags.0?int user:User = auth.Authorization;
---functions---
auth.importAuthorization#a57a7dad id:long bytes:bytes = auth.Authorization;
auth.exportAuthorization#e5bfffcd dc_id:int = auth.ExportedAuthorization;

auth.exportAuthorization must be executed in the current DC (the DC with which a connection has already been established), passing in dc_id as the value for the new DC. The method should return the user identifier and a long string of random data. An import operation can be performed at the new DC by sending it what was received. Queries requiring authorization can then be successfully executed in the new DC.