Operating the Merchant Backend


The following data and facilities have to be set up, in order to run a merchant backend:

  • Serving
  • Currency
  • Database
  • Exchanges
  • Keying
  • Bank account
  • Instances

In this document, we assume that $HOME/.config/taler.conf is being customized.


The merchant backend can serve HTTP over both TCP and UNIX domain socket.

The following values are to be configured under the section [merchant]:

  • SERVE: must be set to tcp to serve HTTP over TCP, or unix to serve HTTP over a UNIX domain socket
  • PORT: set to the TCP port to listen on if SERVE is tcp.
  • UNIXPATH: set to the UNIX domain socket path to listen on if SERVE is unix
  • UNIXPATH_MODE: number giving the mode with the access permission mask for the UNIXPATH (i.e. 660 = rw-rw—-).


The merchant backend supports only one currency. This data is set under the respective option currency in section [taler].


The option db under section [merchant] gets the DB backend’s name the merchant is going to use. So far, only db = postgres is supported. After choosing the backend, it is mandatory to supply the connection string (namely, the database name). This is possible in two ways:

  • via an environment variable: TALER_MERCHANTDB_POSTGRES_CONFIG.
  • via configuration option config, under section [merchantdb-BACKEND]. For example, the demo merchant is configured as follows:
db = postgres

config = postgres:///talerdemo


The options uri and master_key, under section [merchant-exchange-MYEXCHANGE], let the merchant add the exchange MYEXCHANGE among the exchanges the merchant wants to work with. MYEXCHAGE is just a mnemonic name chosen by the merchant (which is not currently used in any computation), and the merchant can add as many exchanges as it is needed. uri is the exchange’s base URL. Please note that a valid uri complies with the following pattern:


master_key is the base32 encoding of the exchange’s master key (see /keys). In our demo, we use the following configuration:

URI = https://exchange.test.taler.net/


The option keyfile under section [merchant-instance-default] is the path to the merchant’s default instance private key. This key is needed to sign certificates and other messages sent to wallets and exchanges. To generate a 100% compatible key, it is recommended to use the gnunet-ecc tool.

Bank account

This piece of information is used when the merchant redeems coins to the exchange. That way, the exchange will know to which bank account it has to transfer real money. The merchant must specify which system it wants receive wire transfers with. We support a test wire format so far, and supporting SEPA is among our priorities.

The wire format is specified in the option wireformat under section [merchant], and the wire details are given via a JSON file, whose path must be indicated in the option X_response_file under section [default-wireformat], where X matches the chosen wireformat. In our demo, we have:

wireformat = test

test_response_file = ${TALER_CONFIG_HOME}/merchant/wire/test.json

The file test.json obeys to the following specification

interface WireDetails {
  // matches wireformat
  type: string;

  // base URL of the merchant's bank
  bank_uri: string;

  // merchant's signature (unused, can be any value)
  signature: string;

  // merchant's account number at the bank
  account_number: Integer;

  // the salt (unused, can be any value)
  salt: any;

As an example, test.json used in our demo is shown below:

"type": "test",
"bank_uri": "https://bank.test.taler.net/",
"account_number": 6,
"salt": "SALT"


In Taler, multiple shops can rely on the same merchant backend. In Taler terminology, each of those shops is called (merchant) instance. Any instance is defined by its private key and its wire details. In order to add the instance X to the merchant backend, we have to add the sections [merchant-instance-X] and [X-wireformat], and edit them as we did for the default instance. For example, in our demo we add the instance Tor as follows:

KEYFILE = ${TALER_DATA_HOME}/merchant/tor.priv


TEST_RESPONSE_FILE = ${TALER_CONFIG_HOME}/merchant/wire/tor.json

Please note that Taler messagging is designed so that the merchant frontend can instruct the backend on which instance has to be used in the various operations. This information is optional, and if not given, the backend will act as the default instance.


In order to compile your merchant backend, you firstly need to install the GNU Taler exchange. As of other dependencies, the merchant backend needs exactly the same ones as the exchange does. Follow those instructions to build everything needed.

Assuming all the dependencies have been correctly installed, we can now build the merchant backend using the following commands:

$ git clone git://taler.net/merchant
$ cd merchant
$ ./bootstrap
$ ./configure [--prefix=PFX] \
              [--with-gnunet=GNUNETPFX] \
$ # Each dependency can be fetched from non standard locations via
$ # the '--with-<LIBNAME>' option. See './configure --help'.
$ make
$ make install