ObjectBox Sync Server

How to use standalone ObjectBox Sync Server and set it up as a data synchronization target for clients.

ObjectBox sync-server comes as a single command-line application and includes:

  • ObjectBox core database - used internally by Sync server and HTTP server

  • ObjectBox Sync server - listens for connections from sync clients

  • ObjectBox HTTP server - simple database browser and management - should be only used by the administrator

All of these are started when you execute the sync-server binary and you can control the configuration using command-line arguments and/or using a configuration JSON file.

Getting started

To start the server you will, at the very least, need to specify the directory path and the database model JSON, as generated by one of the ObjectBox language bindings (Java/Swift/Go/...) or by the objectbox-generator(download here: ObjectBox Generator releases).

The model JSON is used to initialize the database schema and can also be used later to switch the schema to a newer version, by starting the server with an updated model file. Alternatively, the schema version can be changed through the web UI.

Let's say you already have a model file ready in the current folder and try to run the server. The server will refuse to start because it requires some form of client authentication (e.g. shared key).

$ ./sync-server --model=objectbox-model.json
001-13:05:07.3526 [ERROR] [SySvAp] Runtime error: Authenticator must be set before starting
001-13:05:07.4524 [INFO ] [SvSync] Stopped (port 0)
Authenticator must be set before starting

For development you can disable authentication altogether, allowing all the clients that have network connectivity. Warning: don't use this setting in production or on networks you don't trust.

$ ./sync-server --model=objectbox-model.json --unsecure-no-authentication
001-13:09:15.6995 [INFO ] [SySvAp] Default configuration file sync-server-config.js not found
001-13:09:15.6996 [INFO ] [SvrMsg] Registering server transport for ws
001-13:09:15.6997 [INFO ] [SySvAp] Database has no schema, loading from objectbox-model.json
001-13:09:15.6998 [INFO ] [SySvAp] Model file objectbox-model.json loaded, it has 2563 bytes
001-13:09:15.6999 [INFO ] [SchVer] Inserted new schema version ID 1
001-13:09:15.6999 [INFO ] [SySvAp] Switching schema version ID to 1
001-13:09:15.7002 [INFO ] [SchVer] Update schema version ID 1: acceptClients := true
001-13:09:15.7012 [INFO ] [SySvAp] Starting the server with a schema version ID 1
001-13:09:15.7018 [INFO ] [SvrUws] UwsServer listening on all interfaces, port 9999 without SSL
001-13:09:15.7018 [WARN ] [SvSync] INSECURE no-authentication mode enabled - every client allowed without any validation
001-13:09:15.7019 [INFO ] [SvSync] Started on port 9999
001-13:09:15.7019 [INFO ] [SySvAp] Starting object browser on
001-13:09:15.7020 [INFO ] [HttpSv] Running in single-store mode with an already opened store
001-13:09:15.7020 [INFO ] [HttpSv] Listening on
001-13:09:15.7020 [INFO ] [HttpSv] User management: enabled
001-13:09:15.7021 [INFO ] [SySvAp] ObjectBox sync server started in 2.63 ms

As of now, the server is running and accessible:

  • sync on all interfaces, port 9999

  • admin web UI: localhost, port 9980

You can have a look at the admin UI at in your favourite web browser and start using sync in the client apps.


Let's have a look at sync-server --help:

ObjectBox Sync Server
sync-server [OPTION...]
-d, --db-directory arg directory where the database is stored
(default: objectbox)
-b, --bind arg host/IP and port on which the server should
listen on (default: ws://
-c, --conf arg configuration file path (default:
-m, --model arg schema model file to load (JSON) (default: )
--cert arg certificate file path (default: )
--browser-bind arg host/IP and port on which the object-browser
should listen on (default:
--browser-threads arg number of the worker threads to use for the
object-browser (default: 4)
[UNSECURE] allow connections without
authentication (default: false)
--debug enable debug logs (default: false)
-h, --help show help

Alternatively, you can choose to provide the configuration in a sync-server-config.js JSON file:

"dbDirectory": "objectbox",
"modelFile": "",
"bind": "ws://",
"browserBind": "",
"browserThreads": 4,
"certificatePath": "",
"auth": {
"sharedSecret": "",
"google": {
"clientIds": []
  • dbDirectory directory where the database is stored

  • modelFile schema (model) file to create the database with or to use for a schema update

  • bind Sync server will bind on this schema, host and port combination, should be in the form ws://hostname:port for example ws:// You can also bind to a specific IP address on the server machine by providing the exact address, as given by ifconfig or ip addr, e.g. ws://

  • browserBind HTTP server (admin/web UI) will bind on this schema, host and port combination

  • browserThreads number of threads the HTTP server uses - a low number is enough as it's for admins only but you may

    need to increase if running in some cloud setups that keep the connections active (e.g. kubernetes)

  • certificatePath Server SSL certificate directory; SSL will be enabled if not empty; expects cert.pem and key.pem at certPath

  • auth.sharedSecret if not empty, enables the shared secret authentication with the given key

  • auth.google.clientIds a list of GoogleAuth client IDs (strings)

You can omit any fields you're not interested to override them. CLI options have precedence (override) the options in the JSON config file.

Status Page

In the main menu you will find "Status" to open a page with some useful information. While the following layout is still not final, it will give you a first impression of what to expect:

The "Debug logging" switch on the status page enables a very detailed logging (to standard output).


By default only logs with "info" level and above are enabled, which are relatively rare. For example, there's nothing logged about standard interactions with clients. That might be overwhelming with a few hundred clients already. Info logs should never "spam" you, no matter how many clients are connected.

Let's look at a typical log during startup:

001-14:09:07.5792 [INFO ] [SySvAp] Default configuration file sync-server-config.js not found
001-14:09:07.5793 [INFO ] [SvrMsg] Registering server transport for ws
001-14:09:07.5831 [INFO ] [SvrUws] UwsServer listening on all interfaces, port 9999 without SSL
001-14:09:07.5832 [WARN ] [SvSync] INSECURE no-authentication mode enabled - every client allowed without any validation
001-14:09:07.5883 [INFO ] [SvSync] Started on port 9999
001-14:09:07.5884 [INFO ] [SySvAp] Starting object browser on
001-14:09:07.5885 [INFO ] [HttpSv] Running in single-store mode with an already opened store
001-14:09:07.5885 [INFO ] [HttpSv] Listening on
001-14:09:07.5885 [INFO ] [HttpSv] User management: enabled
001-14:09:07.5890 [INFO ] [SySvAp] ObjectBox sync server started in 10 ms

As you can see, logging is structured into the following columns:

  • Thread: the first three digits are the number of the thread that logged the text message

  • Time: UTC time using 24 hours format, including 1/10,000 second precision (1/10 milliseconds)

  • Level: One of the log levels (listed with increasing severity):

    • DEBUG: extensive logs to help diagnosing a specific behavior

    • INFO: "important" information

    • WARN: something unusual has happend that you might want to check.

    • ERROR: reserved for special error occasions ("something bad happend") that typically require some action. It might be that the machine is running out of resources (RAM, disk space etc) or an unexpected situation was encountered. When in doubt, reach out to the ObjectBox team to clarify what's going on.

  • Tag (optional): Most logs include a tag identifying the internal component

  • Message: the actual log text

After startup, you typically won't see anymore logs by default. In contrast to "info" and above, "debug" level logs gives you extensive information including client connects and message interactions with clients.

To give you a feel what debug logs are like have a look at the following example. It shows a new client connecting, logging in and sending data:

014-15:25:18.3188 [DEBUG] [SvrUws] (#1) Connection from
014-15:25:18.3194 [DEBUG] [SvSync] (#1) Got msg of type LOGIN and length 28
014-15:25:18.3195 [DEBUG] [W-Pool] "SrvRead" Submitted new job successfully
002-15:25:18.3195 [DEBUG] [SvLgIn] (D10F) Authenticator 0 welcomes a new client
002-15:25:18.3195 [DEBUG] [W-Pool] "SrvRead" W#1 finished job in 0.06 ms
014-15:25:18.3195 [DEBUG] [SvrUws] Popped 1 messages to send
014-15:25:18.3207 [DEBUG] [SvSync] (D10F) Got msg of type APPLY_TX and length 140
014-15:25:18.3207 [DEBUG] [W-Pool] "SrvWrite" Submitted new job successfully
012-15:25:18.3215 [DEBUG] [TxLogA] Applied 4 commands
012-15:25:18.3277 [DEBUG] TX #5492 committed
009-15:25:18.3277 [DEBUG] [SvLogM] Applied TX log from D10FDB04A0F0: base=5460B3A444C2CD5377EAE8FBFFAC6B12, old=9A37C8BCA4232AA2D6E502AE2D069C15, new=4B2D216711C60497088B5FB10816CC98
009-15:25:18.3277 [DEBUG] [SvApTx] (D10F) Sending ACK_TX #1: 5460B3A4
009-15:25:18.3277 [DEBUG] [W-Pool] "SrvWrite" W#1 finished job in 7.01 ms
013-15:25:18.3278 [DEBUG] [SvDPsh] Have 116 bytes of data (0 clients).
014-15:25:18.3278 [DEBUG] [SvrUws] Popped 1 messages to send

Current limitations

While we hope that ObjectBox Sync is already useful for your use case, we're also aware that some features are missing. We're looking forward to your feedback to prioritize the most requested features.

Depending on your use case, there still might be an important feature missing in the sync version you have received: "ID mapping" allows each sync client (and server) to operate in their own "ID space". This ensures clients won't overwrite each other when using the same IDs (by default, IDs start at 1 in ObjectBox). Workarounds exist until ID mapping is operational; e.g. assign IDs yourself so that they cannot collide; or ensure that in product demos you only insert new objects after sync completed. When in doubt, ask the ObjectBox team.


Please let the ObjectBox team know if you run into issues; we're happy to help. We are aware of rough edges still exist and might be able to help you one way or the other.