Responding to LVAlert Messages

Section author: Reed Essick


This tutorial may not be fully up-to-date. The preferred resource for installing ligo-lvalert and configuring a listener is the ligo-lvalert user guide. However, at present (August 2019), it is not fully completed, and the tutorial on this page may still provide some useful information.

This tutorial will show you how to
  • register to receive LVAlerts
  • subscribe and unsubscribe from pubsub nodes
  • instantiate and manage an lvalert_listen instance
  • interact with GraceDB through the Python REST interface in a script that is launched via lvalert_listen

This tutorial assumes that the ligo-lvalert software package is already installed on your machine (this is true on all cluster machines).

While we attempt to be pedagogically complete as much as possible, we would like to stress that the existing documentation and help-strings for the command-line and Python packages are very useful and should be the final reference if you have any questions.

Registering to receive LVAlert messages

LSC-Virgo members can activate accounts by simply completing the form here.

If you need to create an account that is not attached to your, you can email IGWN Computing Helpdesk and request an account. Once you have created an account, you will be able to subscribe the account to different pubsub nodes and receive lvalert messages.

Subscribing to pubsub nodes

LVAlert messages are broadcast through pubsub nodes and different messages go through different nodes. For instance, all gstlal events created in GraceDB are announced through the pubsub node called:


which includes both highmass and lowmass events. If you instead want to only receive lowmass events, these are announced through:


Importantly, if you subscribe to both cbc_gstlal and cbc_gstlal_lowmass, you will receive two alerts for every gstlal lowmass event. The general format of:


is followed by all pubsub nodes used to announce events and annotations to those events in GraceDB.

Fill out the form and follow all instructions to create an account attached to your “”. For the rest of this tutorial, I will refer to the username as “”, but you should replace this with your own account’s information. You’ll be prompted for your password after each command.

Note: To bypass this, create a .netrc file in your home directory and enter your authentication information:

machine login password passw0rd

With this setup, you won’t need to include the -a flag for your username, or enter your password. Your .netrc file should only be accessible by you, so be sure to do chmod 600 $HOME/.netrc.

To actually subscribe to a pubsub node, we use lvalert_admin which allows you to manage your subscriptions. This includes subscribing to new nodes, unsubscribing from nodes and viewing your current subscriptions. We will now subscribe your account to cbc_gstlal_lowmass. Run:

lvalert_admin -a --subscribe --node cbc_gstlal_lowmass

You can confirm that your account is successfully subscribed to this node by running:

lvalert_admin -a --subscriptions

which will list your account’s subscriptions. You should see cbc_gstlal_lowmass in the resulting list. To unsubscribe from a node, use:

lvalert_admin -a --unsubscribe --node cbc_gstlal_lowmass

but for now we’ll leave our subscription in place. If you’d like to subscribe to other nodes, simply repeat the subscription command and replace cbc_gstlal_lowmass with the name of the node to which you’d like to subscribe. A complete list of nodes is available by running:

lvalert_admin -a --get-nodes

For this tutorial, let’s subscribe to another node to show how things scale. Run:

lvalert_admin -a --subscribe --node cbc_gstlal_highmass

Creating an LVAlert node

Users can create their own LVAlert pubsub nodes as well. Unsurprisingly, this is also straightforward. Simply run:

lvalert_admin -a --create --node

to create a node called Of course, you’ll want to change “” to your account’s name. Go ahead and create this node. If you need to delete it at any time, you can with:

lvalert_admin -a --delete --node

but leave it be for the moment. You now have a node owned by your account to which you can publish alerts. We’ll come back to this when we test our set-up. You will also need to subscribe to this node with:

lvalert_admin -a --subscribe --node


lvalert_admin -a --subscriptions

and make sure you see:


in the output.

Starting and managing an lvalert_listen instance

Now you have an lvalert account and it is subscribed to a few pubsub nodes. It’s time to set up an lvalert_listen instance which allows your code to receive and react to announcements broadcast through the pubsub nodes. The first thing you’ll need is a config file. Using your favorite text editor, create a file called myLVAlertListen.ini with the following as its contents:

executable = /bin/true

executable = /bin/false

executable = /bin/true

Now run:

lvalert_listen -a -c myLVAlertListen.ini > myLVAlertListen.out &

Congratulations! You’ve set up an lvalert_listen instance which reacts to announcements published to the cbc_gstlal_lowmass, cbc_gstlal_highmass and nodes.

Here’s what’s happening: lvalert_listen hears announcements made to any node to which your account is subscribed. When an alert is received, it looks in the config file (loaded into memory) for the associated section. Importantly, if there is no section in the config file corresponding to the pubsub node’s name (an exact match is required), lvalert_listen ignores the announcements from that node even if you are subscribed to it. If it finds a section, it looks for the “executable” option and attempts to run the associated value (in this case /bin/true) via Python’s subprocess module. The delegation to subprocess.Popen does not split the value so this must be a single filename for the executable. If your executable takes in options, we recommend wrapping it in a simple shell script and specifying the shell script within myLVAlertListen.ini. We’ll get to that in a bit.

In this way, you can have multiple lvalert_listen instances for a single account listening to multiple different nodes and doing multiple different things. Furthermore, if you provide multiple sections in myLVAlertListen.ini you can react to announcements from different pubsub nodes in different ways, all within the same lvalert_listen instance.

Right now your listener (running in the background) isn’t doing much. When cbc_gstlal_lowmass alerts are received, it forks an instance of /bin/true and when cbc_gstlal_highmass alerts are received, it forks an instance of /bin/false. We can improve upon that pretty easily.

Let’s start by creating some basic wrapper scripts to print that we’ve received alerts. Again, using your favorite text editor, create the file and fill it with:

echo "received an alert about a cbc_gstlal_lowmass event!" >> lvalert_cbc_gstlal_lowmass.out

Similarly, create and fill it with:

echo "received an alert about a cbc_gstlal_highmass event!" >> lvalert_cbc_gstlal_highmass.out

Finally, create a file for your test node,, which contains:

read a
echo "received a test alert: ${a}" >>

Once you’ve done that, ensure that all three shell scripts are executables (required by the delegation through subprocess.Popen) with:

chmod +x
chmod +x
chmod +x

and edit myLVAlertListen.ini so it reads:

executable = ./

executable = ./

executable = ./

It is generally a good rule of thumb to provide the full paths to executables and output files in both myLVAlertListen.ini as well as these simple shell scripts. However, for the purpose of this tutorial we’ll stick with relative paths.

Now, because you have modified the lvalert_listen.ini file, you’ll need to restart your lvalert_listen instance. Find the PID in the process table, kill the existing process, and restart the listener using the command from above.

You can also specify a resource name in your call to lvalert_listen using the -r flag:

lvalert_listen -a -c myLVAlertListen.ini -r listener1 &

If you don’t specify this parameter, a random UUID is generated for the resource name. The important point to consider is that only one listener can exist for any (, passw0rd, triple anywhere in the network. If you launch a second process with matching values of this triple, one of the processes is killed automatically (although which process dies may not be deterministic). Thus, I can kill processes running at CIT by creating processes at UWM with the same resource name. This can be extremely dangerous and annoying, so please be careful. If you want to directly specify resource names for all of your listener processes, you can do something like:

lvalert_listen -a -c myLVAlertListen.ini -r oneInstance &
lvalert_listen -a -c myLVAlertListen.ini -r twoInstance &

This will launch two instances of lvalert_listen (both using the same config file) with different resource names (note that this can also be achieved by not specifying the resource name at all). They will both react to alerts and fork processes. If each points to a different config file, I can then get multiple types of follow-up processes forked for the same announcement through a single pubsub node.

When alerts are received, you will see a line printed to the associated files. Note, the scripts for the cbc_gstlal nodes do not report anything about the actual alerts received, whereas the script for your test node reads in stdin (to a variable called “a”) and then echo’s that into the output file. This is how lvalert_listen passes the contents of the alert into the forked subprocess, via stdin. We’ll come back to that later when we interact with GraceDB.

For now, let’s test your set-up by publishing a few announcements to your test pubsub node. Create a file called test.txt and fill it with some text like:

just a test announcment

Then run:

lvalert_send -a -n --file test.txt

This publishes the contents of test.txt as a string to the node If your listener is running in the background, then you should see a new line in which reads:

received a test alert: just a test announcement

If you repeat the lvalert_send command, you should see multiple lines appear, one for each time you sent an alert.

Note, each time we change the lvalert_listen config file (myLVAlertListen.ini) we have to restart the listener for the changes to take effect. However, if the config file points to wrapper script we can modify the contents of the wrapper script and have the changes take effect immediately for all future events without restarting the lvalert_listen process. This can be quite handy, although you should be careful to keep track of what was actually run when (version controlling the config file and lvalert-run_*sh scripts is a good idea).

It is worth stressing that you do not have to actually use a wrapper script. If you have an executable that can be called via subprocess in the same way as the wrapper script, then you can simply specify that within myLVAlertListen.ini instead of dealing with wrappers at all. This can reduce the number of files that have to be managed but because of how lvalert_listen forks the executable through subprocess the executable cannot take in any command line options or arguments.

Now, lvalert_listen is a fairly robust process and is unlikely to throw errors or fall over by itself. However, occasionally server-side or local trouble can cause your listener to die and you will need to restart it. Several solutions exist, although the preferred option is Monit which can automatically restart processes and notify you that it did so.

Reacting to GraceDB

Now that you’ve got an lvalert_listen instance running which reacts to a few different pubsub nodes, let’s really dig into the full potential of this system.

So far, we either haven’t used the contents of the alert or have simply printed them into a file. That’s nice, but we can do much better. GraceDB (the main publisher of alerts) sends JSON (JavaScript Object Notation) strings through lvalert and there are several convenient tools to parse these in Python. Similarly, there is an extremely useful RESTful interface to GraceDB implemented in Python, although command-line executables also exist.

Let’s start by mining the JSON string sent by GraceDB for some information. Create a Python executable and fill it with the following:

import json
import sys

alert = json.loads(
print('uid : ' + alert['uid'])

Don’t forget to give this executable permissions with:

chmod +x

Now, modify your wrapper script for the test node ( so it reads:

./ >>

When we send messages to the test node, it will now delegate to We don’t have to restart the lvalert_listen instance because that still points to, which is nice.

Let’s go ahead and send a test message in JSON format. Edit test.txt so it reads:

{"uid": "G12345"}

and run:

lvalert_send -a --node --file test.txt

You should see a new line in which reads:

uid : G12345

Ta-da! You’ve now sent, received, parsed, and reacted to a JSON string through lvalert. This is the key way all follow-up processes listen for events in GraceDB and react accordingly. Note, the command will block until there is something in stdin and this can cause your code to hang if you don’t specify anything. This should not be the case when it is called from within lvalert_listen, but it can sometimes be annoying when debugging your follow-up scripts.

Let’s do something a bit more concrete with more specific examples of how we can interface with GraceDB based off lvalert messages. Open and modify it so it reads:

import json
import sys

from import GraceDb

alert = json.loads(
print('uid : ' + alert['uid'])

gdb = GraceDb() ### instantiate a GraceDB object which connects to the default server

if alert['alert_type'] == 'new': ### the event was just created and this is the first announcment
    gdb.writeLog( alert['uid'], message=" heard an alert about this new event!" )

elif alert['alert_type'] == 'update': ### something happened in GraceDB for this event and GraceDB is letting everyone know
    gdb.writeLog( alert['uid'], message=" heard an alert about an update for this event!" )

Now, if we modify test.txt to:

{"uid": "G12345", "alert_type": "new", "far": 1e-8}

and send it, will try to write a log entry in GraceDB for event G12345. It’s easy to see that you can filter alerts out (e.g.: only react to ‘new’ events) and modify your follow-up processes behavior accordingly. To check that this worked, you’ll need to look at the associated GraceDB page, expand the “full log” section and look for your log message.

IMPORTANTLY, I’ve just made up ‘G12345’ as an example. If you really want to test your script, you should choose a test event from GraceDB. A query for these events is available here. NOTE: please do NOT test your script with important events in GraceDB like G184098 (the cWB entry for GW150914) or others with low FAR. Instead, please use a test event as described above. There are also test instances of GraceDB available if you’d prefer to not work with the production server right away. Contact IGWN Computing Helpdesk with a descriptive subject line for more information.

At this point, you’re pretty much ready to go. However, I’ll leave you with one more example for what might look like:

import json
import sys

from import GraceDb

FarThr = float(sys.argv[1])

alert = json.loads(
print('uid : '+alert['uid'])

gdb = GraceDb() ### instantiate a GraceDB object which connects to the default server

if alert['alert_type'] == 'new': ### the event was just created and this is the first announcment
    if alert['far'] < FarThr:
        with open("iReact.txt", "w") as file_obj:
            print("wow! this was a rare event!  It had FAR = %.3e < %.3e, which was my threshold"%(alert['far'], FarThr), file=file_obj)
        gdb.writeLog( alert['uid'], message=" heard an alert about this new event!", filename="iReact.txt", tagname=["data_quality"] )

Try to figure out exactly what this version does. If you can understand everything within this script you certainly know enough to get your follow-up process running! Hint: to get this to run correctly, you’ll want to modify so it looks like:

./ 1e-8 >>