Re: Distributed queue: how to ensure no lost items?
At least once is generally the case in queuing systems unless you can do a distributed transaction with your consumer. What comes in handy in an at least once case, is letting the consumer know that a message may have 'potentially' already been processed. That way he can double check first before he goes off and processes the message again. But adding that info in ZK might be more expensive that doing the double check every time in consumer anyways. On Thu, Jan 8, 2009 at 11:42 AM, Benjamin Reed br...@yahoo-inc.com wrote: We should expand that section. the current queue recipe guarantees that things are consumed at most once. to guarantee at least the consumer creates an ephemeral node queue-X-inprocess to indicate that the node is being processed. once the queue element has been processed the consumer deletes queue-X and queue-X-inprocess (in that order). using an emphemeral node means that if a consumer crashes, the *-inprocess node will be deleted allowing the queue elements it was working on to be consumed by someone else. putting the *-inprocess nodes at the same level of the queue-X nodes allows the consumer to get the list of queue elements and the inprocess flags with the same getChildren call. the *-inprocess flag ensures that only one consumer is processing a given item. by deleting queue-X before queue-X-inprocess we make sure that no other consumer will see queue-X as available for consumption after it is processed and before it is deleted. this is at last once, because the consumer has a race condition. the consumer may process the item and then crash before it can delete the corresponding queue-X node. ben -Original Message- From: Stuart White [mailto:stuart.whi...@gmail.com] Sent: Thursday, January 08, 2009 7:15 AM To: zookeeper-user@hadoop.apache.org Subject: Distributed queue: how to ensure no lost items? I'm interested in using ZooKeeper to provide a distributed producer/consumer queue for my distributed application. Of course I've been studying the recipes provided for queues, barriers, etc... My question is: how can I prevent packets of work from being lost if a process crashes? For example, following the distributed queue recipe, when a consumer takes an item from the queue, it removes the first item znode under the queue znode. But, if the consumer immediately crashes after removing the item from the queue, that item is lost. Is there a recipe or recommended approach to ensure that no queue items are lost in the event of process failure? Thanks! -- Regards, Hiram Blog: http://hiramchirino.com Open Source SOA http://open.iona.com
RE: Distributed queue: how to ensure no lost items?
That is a good point. you could put a child znode of queue-X that contains the processing history. Like who tried to process and what time they started. ben From: Hiram Chirino [chir...@gmail.com] Sent: Monday, January 12, 2009 8:48 AM To: zookeeper-user@hadoop.apache.org Subject: Re: Distributed queue: how to ensure no lost items? At least once is generally the case in queuing systems unless you can do a distributed transaction with your consumer. What comes in handy in an at least once case, is letting the consumer know that a message may have 'potentially' already been processed. That way he can double check first before he goes off and processes the message again. But adding that info in ZK might be more expensive that doing the double check every time in consumer anyways. On Thu, Jan 8, 2009 at 11:42 AM, Benjamin Reed br...@yahoo-inc.com wrote: We should expand that section. the current queue recipe guarantees that things are consumed at most once. to guarantee at least the consumer creates an ephemeral node queue-X-inprocess to indicate that the node is being processed. once the queue element has been processed the consumer deletes queue-X and queue-X-inprocess (in that order). using an emphemeral node means that if a consumer crashes, the *-inprocess node will be deleted allowing the queue elements it was working on to be consumed by someone else. putting the *-inprocess nodes at the same level of the queue-X nodes allows the consumer to get the list of queue elements and the inprocess flags with the same getChildren call. the *-inprocess flag ensures that only one consumer is processing a given item. by deleting queue-X before queue-X-inprocess we make sure that no other consumer will see queue-X as available for consumption after it is processed and before it is deleted. this is at last once, because the consumer has a race condition. the consumer may process the item and then crash before it can delete the corresponding queue-X node. ben -Original Message- From: Stuart White [mailto:stuart.whi...@gmail.com] Sent: Thursday, January 08, 2009 7:15 AM To: zookeeper-user@hadoop.apache.org Subject: Distributed queue: how to ensure no lost items? I'm interested in using ZooKeeper to provide a distributed producer/consumer queue for my distributed application. Of course I've been studying the recipes provided for queues, barriers, etc... My question is: how can I prevent packets of work from being lost if a process crashes? For example, following the distributed queue recipe, when a consumer takes an item from the queue, it removes the first item znode under the queue znode. But, if the consumer immediately crashes after removing the item from the queue, that item is lost. Is there a recipe or recommended approach to ensure that no queue items are lost in the event of process failure? Thanks! -- Regards, Hiram Blog: http://hiramchirino.com Open Source SOA http://open.iona.com
Maximum number of children
Hello, We are attempting to use ZooKeeper to coordinate daily email thresholds. To do this we created a node hierarchy of /root/template/date/email_hash The idea being that we only send the template to an email address once per day. This is intended to support millions of email hashes per day. From the ZooKeeper perspective we just attempt a create and if it succeeds we proceed and if we get a node exists exception we stop processing. This seems to operate fine for over 2 million email hashes so far in testing. However we also want to prune all previous days nodes to conserve memory. We have run into a hard limit while using the getChildren method for a given /root/template/date. If the List of children exceeds the hardcoded 4,194,304 byte limit ClientCnxn$SendThread.readLength() throws an exception on line 490. So we have an issue that we can not delete a node that has children nor is it possible to delete a node who has children whose total names exceed 4 Mb. Any feedback or guidance is appreciated. Joshua Tuberville
RE: Maximum number of children
To continue with your current design, you could create a trie based on shared hash prefixes. /root/template/date/ 1a5e67/2b45dc /root/template/date/ 1a5e67/3d4a1f /root/template/date/ 3d4a1f/1a5e67 /root/template/date/ 3d4a1f/2b45dc Alternatively, you could use what the maildir mail storage format uses: /root/template/date/ eh/eharmony.com/jo/joshuatuberville Just check with the second one that all of the characters you support in email addresses are supported in znode names. Thanks, Stu -Original Message- From: Joshua Tuberville joshuatubervi...@eharmony.com Sent: Monday, January 12, 2009 7:53pm To: 'zookeeper-user@hadoop.apache.org' zookeeper-user@hadoop.apache.org Subject: Maximum number of children Hello, We are attempting to use ZooKeeper to coordinate daily email thresholds. To do this we created a node hierarchy of /root/template/date/email_hash The idea being that we only send the template to an email address once per day. This is intended to support millions of email hashes per day. From the ZooKeeper perspective we just attempt a create and if it succeeds we proceed and if we get a node exists exception we stop processing. This seems to operate fine for over 2 million email hashes so far in testing. However we also want to prune all previous days nodes to conserve memory. We have run into a hard limit while using the getChildren method for a given /root/template/date. If the List of children exceeds the hardcoded 4,194,304 byte limit ClientCnxn$SendThread.readLength() throws an exception on line 490. So we have an issue that we can not delete a node that has children nor is it possible to delete a node who has children whose total names exceed 4 Mb. Any feedback or guidance is appreciated. Joshua Tuberville
Re: Maximum number of children
I was going to suggest bucketing with predifined hashes. /root/template/data/hashbucket/hash For the issue raised by Joshua regarding the length of the output from the server -- This is a bug. We seem to allow any number of children ( int) of a node and the getchildren call fails to return the children. This leads to a chicken and egg problem on how to get rid of the nodes if you do not know them. Here we arent saving nething since the server has already processed the request and sent us the data. We should get rid of this hard coded limit. I am not sure why we had this limit. Can you open a jira for this Joshua? thanks mahadev On 1/12/09 5:39 PM, Stu Hood stuh...@mailtrust.com wrote: To continue with your current design, you could create a trie based on shared hash prefixes. /root/template/date/ 1a5e67/2b45dc /root/template/date/ 1a5e67/3d4a1f /root/template/date/ 3d4a1f/1a5e67 /root/template/date/ 3d4a1f/2b45dc Alternatively, you could use what the maildir mail storage format uses: /root/template/date/ eh/eharmony.com/jo/joshuatuberville Just check with the second one that all of the characters you support in email addresses are supported in znode names. Thanks, Stu -Original Message- From: Joshua Tuberville joshuatubervi...@eharmony.com Sent: Monday, January 12, 2009 7:53pm To: 'zookeeper-user@hadoop.apache.org' zookeeper-user@hadoop.apache.org Subject: Maximum number of children Hello, We are attempting to use ZooKeeper to coordinate daily email thresholds. To do this we created a node hierarchy of /root/template/date/email_hash The idea being that we only send the template to an email address once per day. This is intended to support millions of email hashes per day. From the ZooKeeper perspective we just attempt a create and if it succeeds we proceed and if we get a node exists exception we stop processing. This seems to operate fine for over 2 million email hashes so far in testing. However we also want to prune all previous days nodes to conserve memory. We have run into a hard limit while using the getChildren method for a given /root/template/date. If the List of children exceeds the hardcoded 4,194,304 byte limit ClientCnxn$SendThread.readLength() throws an exception on line 490. So we have an issue that we can not delete a node that has children nor is it possible to delete a node who has children whose total names exceed 4 Mb. Any feedback or guidance is appreciated. Joshua Tuberville
