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QUESTION
I've always heard that Spark is 100x faster than classic Map Reduce frameworks like Hadoop. But recently I'm reading that this is only true if RDDs are cached, which I thought was always done but instead requires the explicit cache () method.
I would like to understand how all produced RDDs are stored throughout the work. Suppose we have this workflow:
- I read a file -> I get the RDD_ONE
- I use the map on the RDD_ONE -> I get the RDD_TWO
- I use any other transformation on the RDD_TWO
QUESTIONS:
if I don't use cache () or persist () is every RDD stored in memory, in cache or on disk (local file system or HDFS)?
if RDD_THREE depends on RDD_TWO and this in turn depends on RDD_ONE (lineage) if I didn't use the cache () method on RDD_THREE Spark should recalculate RDD_ONE (reread it from disk) and then RDD_TWO to get RDD_THREE?
Thanks in advance.
...ANSWER
Answered 2021-Jun-09 at 06:13In spark there are two types of operations: transformations and actions. A transformation on a dataframe will return another dataframe and an action on a dataframe will return a value.
Transformations are lazy, so when a transformation is performed spark will add it to the DAG and execute it when an action is called.
Suppose, you read a file into a dataframe, then perform a filter, join, aggregate, and then count. The count operation which is an action will actually kick all the previous transformation.
If we call another action(like show) the whole operation is executed again which can be time consuming. So, if we want not to run the whole set of operation again and again we can cache the dataframe.
Few pointers you can consider while caching:
- Cache only when the resulting dataframe is generated from significant transformation. If spark can regenerate the cached dataframe in few seconds then caching is not required.
- Cache should be performed when the dataframe is used for multiple actions. If there are only 1-2 actions on the dataframe then it is not worth saving that dataframe in memory.
QUESTION
Over many years I've struggled with this same issue. I cannot seem to work out how to use a JavaScript library from TypeScript, reliably.
I seem to get it working by accident and then move on and not revisit such code for years until a extrinsic change forces a breakage, like today when I updated VS 2019.
I've spent days reading about modules and requires and loaders, but I get more and more confused.
Example. I want to use DayJS in a TypeScript .ts
file I am writing.
Here's the sample code.
...ANSWER
Answered 2021-Jun-04 at 18:38I share many of the same frustrations! It's so hard to get Typescript working nicely with Javascript and the microsoft documentation is so obtuse!
In your case : the path to a library is always looked for in node_modules
so in that case you don't need to add the full path.
You also never need to import
a .d.ts
file. You can just put the .d.ts
file somewhere in your working folder and VS Code will detect it.
If you have the .d.ts
file for moment.js
, you will get type completion in VS Code. You don't need to import moment.js
when you load it with a
QUESTION
I am trying to upgrade a Django Application running Celery from Amazon Elastic Beanstalk Linux 1 on running Python 3.6 to Amazon Linux 2 using Python 3.8.
I am having trouble with the Celery application.
In Linux 1 I the following file
...ANSWER
Answered 2021-Apr-07 at 07:16The supervisor is not present in Amazon Linux2 by default.You just have to rewrite this script to run celery under the supervision of systems like this
QUESTION
I am trying to change some information in a textfile. Initially, the textfile looks like
...ANSWER
Answered 2021-May-20 at 14:25The easiest way(that I can think is) to do is open the file
read the file and save the lines
and then empty the file
QUESTION
I've written a very easy wrapper around GDAL in R. It utilises a prewritten statement which is passed to system, creating an output, which I then want to read into the R environment again.
It works by creates a temporary directory in the working directory, printing out an ESRI shape file of our area of interest, and then cuts a raster by this, with some preset information.
My problem: after successfully running the system() call and creating the output file, the function stops. It doesn't execute the next call and read the output into the R environment.
...ANSWER
Answered 2021-May-05 at 15:42The function did not return anything. Here is a somewhat improved version of your function. If you want to keep the output it would make more sense to provide a full path, rather then saving it to a temp folder. I also note that you are not using the argument source_srs
QUESTION
I start to develop with KSQLDB and alongside with Kafka-connect. Kafka-connect is awesome and everything is well and has the behaviour to not reread the records if it detects that it was already read in the past (extrem useful for production). But, for development and debugging of KSQLDB queries it is necessary to replay the data, as ksqldb will create table entries on the fly on emitted changes. If nothing is replayed the to 'test' query stays empty. Any advice how to replay a csv file with kafka connect after the file is inserted for the first time? Maybe, ksqldb has the possiblity to reread the whole topic after the table is created. Has someone the answer for a beginner?
...ANSWER
Answered 2021-Apr-27 at 12:59Create the source connector with a different name, or give the CSV file a new name. Both should cause it to be re-read.
QUESTION
I've been struggling for a while trying to pass a variable between two processes,
I have two processes, one that loops to run a PyQt5 GUI (process A), and a second running some computer vision functions on a video stream in real-time through a while loop (process B),
I want to read a constantly updated (30 times a second) variable from B to A, latency up to ~200ms doesn't matter too much.
Process A will make changes to the GUI based on the var passed from Process B but I'm struggling to pass that variable across,
I've attached some skeleton code that shows my current broken attempt, I have reread the docs several times and a load of questions on here but I'm new to multiprocessing so I'm a bit stumped, I would really appreciate if someone could take a look and point me in the right direction, Thanks!
...ANSWER
Answered 2021-Apr-26 at 19:33What is wrong with your program? It appears to work fine - from parameter passing point of view at least. I tweaked your code a bit (added a random choice and added sleep to the main process to avoid it exiting right away) and it seems to be doing fine:
QUESTION
I read code of C library and can not understand what is going on:
...ANSWER
Answered 2021-Apr-09 at 21:30Yes, it's undefined behavior from the standpoint of standard C - or at least, the value of foo
is indeterminate and can't be safely used for anything.
C17 (n2176) 7.13.2.1 (3):
All accessible objects have values, and all other components of the abstract machine have state, as of the time the longjmp function was called, except that the values of objects of automatic storage duration that are local to the function containing the invocation of the corresponding setjmp macro that do not have volatile-qualified type and have been changed between the setjmp invocation and longjmp call are indeterminate.
That's precisely the situation here: foo
has automatic storage duration and is local to the function where setjmp
is invoked, it's not volatile
-qualified, and it is changed between setjmp
and longjmp
.
The authors of the code were probably thinking that passing &foo
to lib_var
would ensure that it is allocated in memory, not in a register, and that since the compiler couldn't know whether lib_var
would modify foo
, it couldn't constant-propagate the value NULL
into the else
block. But as you say, LTO or various other optimizations could break this. It's possible the authors intended only to support a particular compiler that they knew didn't do any such thing, or that provided some stronger guarantees beyond what the standard says, but it's also possible they were just confused.
The correct fix would be to declare foo
as volatile
, i.e. struct Foo * volatile foo = NULL;
.
QUESTION
SMB2 CHANGE_NOTIFY looks promising, as if it could deliver enough information on subdirectory or subtree updates from the server, so we can keep our listing of remote directory up-to-date by handling the response.
However, it's not a subscription to an event stream, just a one-off command receiving one response, so I suspect that it can be used only as a mere hint to invalidate our cache and reread the directory. When we receive a response, there could be any additional changes before we send another CHANGE_NOTIFY request, and we'll miss the details of these changes.
Is there any way around this problem? Or is rereading directory on learning that it's updated a necessary step?
I want to understand possible solutions on the protocol level (you can imagine I'm using a customized client that I can make do what I want, with some common servers like Windows or smbd3).
...ANSWER
Answered 2021-Mar-26 at 17:36Strictly speaking, not even re-reading the directory listing should save you, since the directory can change between re-reading the listing and submitting another CHANGE_NOTIFY request. The race condition just moves to a different spot.
Except there is no race condition.
This took a little digging, but it’s all there in the specification. In MS-SMB2 v20200826 §3.3.5.19 ‘Receiving an SMB2 CHANGE_NOTIFY Request’, it is stated:
The server MUST process a change notification request in the object store as specified by the algorithm in section 3.3.1.3.
In §3.3.1.3 ‘Algorithm for Change Notifications in an Object Store’, we have:
The server MUST implement an algorithm that monitors for changes on an object store. The effect of this algorithm MUST be identical to that used to offer the behavior specified in [MS-CIFS] sections 3.2.4.39 and 3.3.5.59.4.
And in MS-CIFS v20201001 §3.3.5.59.4 ‘Receiving an NT_TRANSACT_NOTIFY_CHANGE Request’ there is this:
If the client has not issued any NT_TRANSACT_NOTIFY_CHANGE Requests on this FID previously, the server SHOULD allocate an empty change notification buffer and associate it with the open directory. The size of the buffer SHOULD be at least equal to the MaxParameterCount field in the SMB_COM_NT_TRANSACT Request (section 2.2.4.62.1) used to transport the NT_TRANSACT_NOTIFY_CHANGE Request. If the client previously issued an NT_TRANSACT_NOTIFY_CHANGE Request on this FID, the server SHOULD already have a change notification buffer associated with the FID. The change notification buffer is used to collect directory change information in between NT_TRANSACT_NOTIFY_CHANGE (section 2.2.7.4) calls that reference the same FID.
Emphasis mine. This agrees with how Samba implements it (the fsp
structure persists between individual requests). I wouldn’t expect Microsoft to do worse than that by not keeping the promise they made in their own specification.
QUESTION
I am trying to print a custom help message for a bash script. Inside the script, I call a function that then uses python to parse the script, find the functions and the function help string, and then print them. This all works. But, when I try to sort the list of tuples that contains the function name and help string, the sort seems to be ignored. Similar code works as expected in a pure python environment.
Edit: Just noticed I tried the sort two different ways. AFAIK either should have sorted the list, but neither worked.
Edit again: see the accepted answer below for code that actually works. I need to remember to reread my problem code in the morning ;)
...ANSWER
Answered 2021-Mar-19 at 16:30You need to get all the functions, then sort and print :
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