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From Lahiru Gunathilake <glah...@gmail.com>
Subject Re: Zookeeper in Airavata to achieve reliability
Date Mon, 16 Jun 2014 15:14:06 GMT
Hi Supun,

So your suggestion is to create a znode for each thrift service we have and
when the request comes that node gets modified with input data for that
request and thrift service is having a watch for that node and it will be
notified because of the watch and it can read the input from zookeeper and
invoke the operation?

Lahiru


On Thu, Jun 12, 2014 at 11:50 PM, Supun Kamburugamuva <supun06@gmail.com>
wrote:

> Hi all,
>
> Here is what I think about Airavata and ZooKeeper. In Airavata there are
> many components and these components must be stateless to achieve
> scalability and reliability.Also there must be a mechanism to communicate
> between the components. At the moment Airavata uses RPC calls based on
> Thrift for the communication.
>
> ZooKeeper can be used both as a place to hold state and as a communication
> layer between the components. I'm involved with a project that has many
> distributed components like AIravata. Right now we use Thrift services to
> communicate among the components. But we find it difficult to use RPC calls
> and achieve stateless behaviour and thinking of replacing Thrift services
> with ZooKeeper based communication layer. So I think it is better to
> explore the possibility of removing the Thrift services between the
> components and use ZooKeeper as a communication mechanism between the
> services. If you do this you will have to move the state to ZooKeeper and
> will automatically achieve the stateless behaviour in the components.
>
> Also I think trying to make ZooKeeper optional is a bad idea. If we are
> trying to integrate something fundamentally important to architecture as
> how to store state, we shouldn't make it optional.
>
> Thanks,
> Supun..
>
>
> On Thu, Jun 12, 2014 at 10:57 PM, Shameera Rathnayaka <
> shameerainfo@gmail.com> wrote:
>
>> Hi Lahiru,
>>
>> As i understood,  not only reliability , you are trying to achieve some
>> other requirement by introducing zookeeper, like health monitoring of the
>> services, categorization with service implementation etc ... . In that
>> case, i think we can get use of zookeeper's features but if we only focus
>> on reliability, i have little bit of concern, why can't we use clustering +
>> LB ?
>>
>> Yes it is better we add Zookeeper as a prerequisite if user need to use
>> it.
>>
>> Thanks,
>>  Shameera.
>>
>>
>> On Thu, Jun 12, 2014 at 5:19 AM, Lahiru Gunathilake <glahiru@gmail.com>
>> wrote:
>>
>>> Hi Gagan,
>>>
>>> I need to start another discussion about it, but I had an offline
>>> discussion with Suresh about auto-scaling. I will start another thread
>>> about this topic too.
>>>
>>> Regards
>>> Lahiru
>>>
>>>
>>> On Wed, Jun 11, 2014 at 4:10 PM, Gagan Juneja <gagandeepjuneja@gmail.com
>>> >
>>> wrote:
>>>
>>> > Thanks Lahiru for pointing to nice library, added to my dictionary :).
>>> >
>>> > I would like to know how are we planning to start multiple servers.
>>> > 1. Spawning new servers based on load? Some times we call it as auto
>>> > scalable.
>>> > 2. To make some specific number of nodes available such as we want 2
>>> > servers to be available at any time so if one goes down then I need to
>>> > spawn one new to make available servers count 2.
>>> > 3. Initially start all the servers.
>>> >
>>> > In scenario 1 and 2 zookeeper does make sense but I don't believe
>>> existing
>>> > architecture support this?
>>> >
>>> > Regards,
>>> > Gagan
>>> > On 12-Jun-2014 1:19 am, "Lahiru Gunathilake" <glahiru@gmail.com>
>>> wrote:
>>> >
>>> >> Hi Gagan,
>>> >>
>>> >> Thanks for your response. Please see my inline comments.
>>> >>
>>> >>
>>> >> On Wed, Jun 11, 2014 at 3:37 PM, Gagan Juneja <
>>> gagandeepjuneja@gmail.com>
>>> >> wrote:
>>> >>
>>> >>> Hi Lahiru,
>>> >>> Just my 2 cents.
>>> >>>
>>> >>> I am big fan of zookeeper but also against adding multiple hops
in
>>> the
>>> >>> system which can add unnecessary complexity. Here I am not able
to
>>> >>> understand the requirement of zookeeper may be I am wrong because
of
>>> less
>>> >>> knowledge of the airavata system in whole. So I would like to discuss
>>> >>> following point.
>>> >>>
>>> >>> 1. How it will help us in making system more reliable. Zookeeper
is
>>> not
>>> >>> able to restart services. At max it can tell whether service is
up
>>> or not
>>> >>> which could only be the case if airavata service goes down
>>> gracefully and
>>> >>> we have any automated way to restart it. If this is just matter
of
>>> routing
>>> >>> client requests to the available thrift servers then this can be
>>> achieved
>>> >>> with the help of load balancer which I guess is already there in
>>> thrift
>>> >>> wish list.
>>> >>>
>>> >> We have multiple thrift services and currently we start only one
>>> instance
>>> >> of them and each thrift service is a stateless service. To keep the
>>> high
>>> >> availability we have to start multiple instances of them in production
>>> >> scenario. So for clients to get an available thrift service we can use
>>> >> zookeeper znodes to represent each available service. There are some
>>> >> libraries which is doing similar[1] and I think we can use them
>>> directly.
>>> >>
>>> >>> 2. As far as registering of different providers is concerned do
you
>>> >>> think for that we really need external store.
>>> >>>
>>> >> Yes I think so, because its light weight and reliable and we have to
>>> do
>>> >> very minimal amount of work to achieve all these features to Airavata
>>> >> because zookeeper handle all the complexity.
>>> >>
>>> >>> I have seen people using zookeeper more for state management in
>>> >>> distributed environments.
>>> >>>
>>> >> +1, we might not be the most effective users of zookeeper because all
>>> of
>>> >> our services are stateless services, but my point is to achieve
>>> >> fault-tolerance we can use zookeeper and with minimal work.
>>> >>
>>> >>>  I would like to understand more how can we leverage zookeeper in
>>> >>> airavata to make system reliable.
>>> >>>
>>> >>>
>>> >> [1]https://github.com/eirslett/thrift-zookeeper
>>> >>
>>> >>
>>> >>
>>> >>> Regards,
>>> >>> Gagan
>>> >>> On 12-Jun-2014 12:33 am, "Marlon Pierce" <marpierc@iu.edu>
wrote:
>>> >>>
>>> >>>> Thanks for the summary, Lahiru. I'm cc'ing the Architecture
list for
>>> >>>> additional comments.
>>> >>>>
>>> >>>> Marlon
>>> >>>>
>>> >>>> On 6/11/14 2:27 PM, Lahiru Gunathilake wrote:
>>> >>>> > Hi All,
>>> >>>> >
>>> >>>> > I did little research about Apache Zookeeper[1] and how
to use it
>>> in
>>> >>>> > airavata. Its really a nice way to achieve fault tolerance
and
>>> >>>> reliable
>>> >>>> > communication between our thrift services and clients.
Zookeeper
>>> is a
>>> >>>> > distributed, fault tolerant system to do a reliable communication
>>> >>>> between
>>> >>>> > distributed applications. This is like an in-memory file
system
>>> which
>>> >>>> has
>>> >>>> > nodes in a tree structure and each node can have small
amount of
>>> data
>>> >>>> > associated with it and these nodes are called znodes. Clients
can
>>> >>>> connect
>>> >>>> > to a zookeeper server and add/delete and update these znodes.
>>> >>>> >
>>> >>>> >   In Apache Airavata we start multiple thrift services
and these
>>> can
>>> >>>> go
>>> >>>> > down for maintenance or these can crash, if we use zookeeper
to
>>> store
>>> >>>> these
>>> >>>> > configuration(thrift service configurations) we can achieve
a very
>>> >>>> reliable
>>> >>>> > system. Basically thrift clients can dynamically discover
>>> available
>>> >>>> service
>>> >>>> > by using ephemeral znodes(Here we do not have to change
the
>>> generated
>>> >>>> > thrift client code but we have to change the locations
we are
>>> invoking
>>> >>>> > them). ephemeral znodes will be removed when the thrift
service
>>> goes
>>> >>>> down
>>> >>>> > and zookeeper guarantee the atomicity between these operations.
>>> With
>>> >>>> this
>>> >>>> > approach we can have a node hierarchy for multiple of airavata,
>>> >>>> > orchestrator,appcatalog and gfac thrift services.
>>> >>>> >
>>> >>>> > For specifically for gfac we can have different types of
services
>>> for
>>> >>>> each
>>> >>>> > provider implementation. This can be achieved by using
the
>>> >>>> hierarchical
>>> >>>> > support in zookeeper and providing some logic in gfac-thrift
>>> service
>>> >>>> to
>>> >>>> > register it to a defined path. Using the same logic orchestrator
>>> can
>>> >>>> > discover the provider specific gfac thrift service and
route the
>>> >>>> message to
>>> >>>> > the correct thrift service.
>>> >>>> >
>>> >>>> > With this approach I think we simply have write some client
code
>>> in
>>> >>>> thrift
>>> >>>> > services and clients and zookeeper server installation
can be
>>> done as
>>> >>>> a
>>> >>>> > separate process and it will be easier to keep the Zookeeper
>>> server
>>> >>>> > separate from Airavata because installation of Zookeeper
server
>>> little
>>> >>>> > complex in production scenario. I think we have to make
sure
>>> >>>> everything
>>> >>>> > works fine when there is no Zookeeper running, ex:
>>> >>>> enable.zookeeper=false
>>> >>>> > should works fine and users doesn't have to download and
start
>>> >>>> zookeeper.
>>> >>>> >
>>> >>>> >
>>> >>>> >
>>> >>>> > [1]http://zookeeper.apache.org/
>>> >>>> >
>>> >>>> > Thanks
>>> >>>> > Lahiru
>>> >>>>
>>> >>>>
>>> >>
>>> >>
>>> >> --
>>> >> System Analyst Programmer
>>> >> PTI Lab
>>> >> Indiana University
>>> >>
>>> >
>>>
>>>
>>> --
>>> System Analyst Programmer
>>> PTI Lab
>>> Indiana University
>>>
>>
>>
>>
>> --
>> Best Regards,
>> Shameera Rathnayaka.
>>
>> email: shameera AT apache.org , shameerainfo AT gmail.com
>> Blog : http://shameerarathnayaka.blogspot.com/
>>
>
>
>
> --
> Supun Kamburugamuva
> Member, Apache Software Foundation; http://www.apache.org
> E-mail: supun06@gmail.com;  Mobile: +1 812 369 6762
> Blog: http://supunk.blogspot.com
>
>


-- 
System Analyst Programmer
PTI Lab
Indiana University

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