Hi,

I want to download 16S sequences (if available) of thousands of organisms. I use following command: 
```esearch -db nucleotide -query "Abacoproeces saltuum [ORGN] AND 16S" | efetch -format fasta > Abacoproeces_saltuum.txt```
```esearch -db nucleotide -query "Abax carinatus [ORGN] AND 16S" | efetch -format fasta > Abax_carinatus.txt```

...

and this for ~40 000 species. The format for all species is the same (as shown above). 

The command starts running and files are generated. Some contain data and some do not (if sequence is not available).

After searching some (~20-30) species I receive the error below. 

However, the process continues and new files are being downloaded/generated even after the error. Also files with data/sequences are generated. Also the error appears to return multiple times.

Other forum topics regarding this error do not show this behavior, therefore I open this new post regarding this topic. 

My questions: what causes this problem? Somewhere in the error I can read " Too Many Requests", but as I execute it as a bash script, each line should be read seperately. I therefore cannot imagine that the ~40 000 entries (and thus ~40 000 lines) should be the problem. Also I don't think it is a problem regarding the installation of `esearch/efetch/entrez` as some searches work perfectly.

**Can I expect that the files that are generated are correct?** If no data is generated in a file (0 kb file) can I assume that there was really no sequence of that species available, or can the file be empty because of the error?

Thanks.

PC: Ubuntu 18.04.3 LTS

    '_headers' => bless( {
                                            'content-security-policy' => 'upgrade-insecure-requests',
                                                                   'user-agent' => 'libwww-perl/6.31',
                                            'retry-after' => '2',
                                            'content-type' => 'application/json',
                                            'server' => 'Finatra',
                                                              'x-ratelimit-remaining' => 'X-RateLimit-Remaining',
                                            'client-ssl-socket-class' => 'IO::Socket::SSL',
                                                              'client-response-num' => 'Client-Response-Num',
                                                              'client-response-num' => 'Client-Response-Num',
                                                                                     'if-ssl-cert-subject' => 'If-SSL-Cert-Subject'
                                            '_headers' => bless( {
                                                                                     'if-ssl-cert-subject' => 'If-SSL-Cert-Subject'
                                            'content-type' => 'application/json',
                                            'client-ssl-socket-class' => 'IO::Socket::SSL',
                                                              'client-ssl-cipher' => 'Client-SSL-Cipher',
                                                              'content-security-policy' => 'Content-Security-Policy',
                                            'content-type' => 'application/json',
                                            'vary' => 'Accept-Encoding',
                                            'retry-after' => '2',
                                            'content-type' => 'application/json',
                                                                                     'if-ssl-cert-subject' => 'If-SSL-Cert-Subject'
                                            '::std_case' => {
                                            '_uri' => $VAR1->{'_request'}{'_uri_canonical'},
                                            '_headers' => bless( {
                                                                   '::std_case' => {
                                                                                     'if-ssl-cert-subject' => 'If-SSL-Cert-Subject'
                                                                                   },
                                                                   'content-type' => 'application/x-www-form-urlencoded',
                                                                   'user-agent' => 'libwww-perl/6.31'
                                                                 }, 'HTTP::Headers' )
                                          }, 'HTTP::Request' ),
                     '_content' => '{"error":"API rate limit exceeded","api-key":"2a02:a212:1880:6a00:b89d:8514:d405:3f62","count":"4","limit":"3"}
    ',
                     '_msg' => 'Too Many Requests',
                     '_rc' => 429,
                     '_headers' => bless( {
                                            'content-length' => '112',
                                            'server' => 'Finatra',
                                            'client-ssl-cert-subject' => '/C=US/ST=Maryland/L=Bethesda/O=National Library of Medicine/OU=National Center for Biotechnology Information/CN=*.ncbi.nlm.nih.gov',
                                            'client-date' => 'Tue, 27 Oct 2020 12:12:21 GMT',
                                            'client-ssl-socket-class' => 'IO::Socket::SSL',
                                            'content-security-policy' => 'upgrade-insecure-requests',
                                            'date' => 'Tue, 27 Oct 2020 12:12:19 GMT',
                                            'x-ratelimit-limit' => '3',
                                            'x-ua-compatible' => 'IE=Edge',
                                            '::std_case' => {
                                                              'x-ua-compatible' => 'X-UA-Compatible',
                                                              'x-ratelimit-limit' => 'X-RateLimit-Limit',
                                                              'client-ssl-cert-issuer' => 'Client-SSL-Cert-Issuer',
                                                              'strict-transport-security' => 'Strict-Transport-Security',
                                                              'client-peer' => 'Client-Peer',
                                                              'client-response-num' => 'Client-Response-Num',
                                                              'client-date' => 'Client-Date',
                                                              'client-ssl-socket-class' => 'Client-SSL-Socket-Class',
                                                              'content-security-policy' => 'Content-Security-Policy',
                                                              'x-ratelimit-remaining' => 'X-RateLimit-Remaining',
                                                              'client-ssl-cert-subject' => 'Client-SSL-Cert-Subject',
                                                              'client-ssl-cipher' => 'Client-SSL-Cipher',
                                                              'x-xss-protection' => 'X-XSS-Protection'
                                                            },
                                            'client-peer' => '2607:f220:41e:4290::110:443',
                                            'strict-transport-security' => 'max-age=31536000; includeSubDomains; preload',
                                            'client-response-num' => 1,
                                            'vary' => 'Accept-Encoding',
                                            'x-ratelimit-remaining' => '0',
                                            'x-xss-protection' => '1; mode=block',
                                            'client-ssl-cipher' => 'ECDHE-RSA-AES256-GCM-SHA384',
                                            'connection' => 'close',
                                            'content-type' => 'application/json',
                                            'retry-after' => '2',
                                            'client-ssl-cert-issuer' => '/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA'
                                          }, 'HTTP::Headers' )
                   }, 'HTTP::Response' );
    
    WebEnv value not found in search output - WebEnv1 
    Db value not found in fetch input


Have you signed up for [NCBI_API_KEY][1]? If not you should do that first. You should also consider adding a pause after a set number of queries, if you are doing a large number such as 40,000. 

You may also want to consider downloading `nt` blast indexes and pulling out sequence you need from it using `blastdbcmd` locally.


  [1]: https://ncbiinsights.ncbi.nlm.nih.gov/2017/11/02/new-api-keys-for-the-e-utilities/