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Navigation: DEDIS :: Cothority :: Applications :: EventLog


The EventLog (EL) service is for logging events into ByzCoin. Contrary to ordinary event logging services, we offer better security and auditability. Below are some of the main features that sets us apart.

  • Collective witness - a collection of nodes, or conodes, independently observe the logging of an event. The event will only be accepted if a 2/3-majority think it is valid, e.g., the timestamp is reasonable, the client is authorised and so on.
  • Distributed access control - fine-grained client access control with delegation support is configured using DARC.
  • Configurable acceptance criteria - we execute a smart-contract on all nodes, nodes only accept the event if the smart-contract returns a positive result.
  • Existance proof - once an event is logged, an authorised client can request a cryptographic proof (powered by trie) that the event is indeed stored in the blockchain and has not been tampered.

Running the service

The EL service is built into conodes. For the general information about running a conode, please see the conode documentation.

Client API

We offer three ways for clients to connect to the EL service. All the APIs expect an existing ByzCoin object that has a Darc with "spawn:eventlog" and "invoke:eventlog" in its rules. The eventlog signer (which we will create below) must be authorised to use these rules.

An example transcript of correctly setting up and using an Eventlog is:

# Make the ByzCoin instance

$ bcadmin c -roster ../../conode/public.toml
Created ByzCoin with ID 7ad741d44e216fc4475da60b8656b904937639415ec27f7003e13408d6e0510c.
export BC="/Users/jallen/Library/Application Support/bc/data/bc-7ad741d44e216fc4475da60b8656b904937639415ec27f7003e13408d6e0510c.cfg"
$ export BC="/Users/jallen/Library/Application Support/bc/data/bc-7ad741d44e216fc4475da60b8656b904937639415ec27f7003e13408d6e0510c.cfg"

# Make a keypair for the new eventlog, give it permissions on the genesis darc.
# The private key is stored in the el configuration directory.
$ el key
$ bcadmin darc rule -rule spawn:eventlog -identity ed25519:2a53df71edad603e56477d33e82d675a3499ba4719f809fabea95ce546c16b5f
$ bcadmin darc rule -rule invoke:eventlog.log -identity ed25519:2a53df71edad603e56477d33e82d675a3499ba4719f809fabea95ce546c16b5f

# Check the persmissions on the genesis darc.
$ bcadmin darc show

# Make the new eventlog.
$ ./el create -sign ed25519:2a53df71edad603e56477d33e82d675a3499ba4719f809fabea95ce546c16b5f
export EL=b9a6c3868b01e19f6d3d0f62c881582d5a5bd98046dd4e4274b579fd6e66b643
$ export EL=b9a6c3868b01e19f6d3d0f62c881582d5a5bd98046dd4e4274b579fd6e66b643

# Use the eventlog.
$ ./el log -sign ed25519:2a53df71edad603e56477d33e82d675a3499ba4719f809fabea95ce546c16b5f -topic "hello" -content "world"
$ ./el search


The detailed API can be found on godoc. You may find example usage in api_test.go.

Java API

In java, you need to construct a EventLogInstance class. There are two ways to initialise it, the first for when you do not have an existing eventlog instance on ByzCoin to connect to, the other is when you do. Below we give a general overview. Please see the Java docs for more information.

// Create the eventlog instance. It expects a ByzCoin RPC, 
// the darc ID that has the "spawn:eventlog" rule, a list of 
// signers that are authorized to in the "spawn:eventlog" rule
// and the counters (used for preventing replay attacks).
// You can get the counters using bcRPC.getSignerCounters.
EventLogInstance el = new EventLogInstance(bcRPC, darcID, admins, signerCounters);

If you would like to connect to the same instance, you need to save the result of el.getInstanceId() and the ByzCoin RPC. Then use EventLogInstance.fromByzcoin(ByzCoinRPC bc, InstanceId id) to connects to an existing instance.

It's straightforward to log events, as long as the event is correctly signed. The signer must be the one with the "invoke:eventlog.log" permission.

Event event = new Event("login", "alice");
InstanceId key = this.el.log(event, signers, signerCounters);
// wait for the block to be added
Event event2 = this.el.get(key);

We also have a search API, which allows searching for a particular topic within a time-range.

long now = System.currentTimeMillis() * 1000 * 1000;
SearchResponse resp ="", now - 1000, now + 1000);


Please see the el documentation here.

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